Method device for recording information, and method for measuring the transmittance of liquid crystals, which is applied to recording information

ABSTRACT

The invention provides a system in which a photoelectric sensor having a transparent electrode and a photoconductive layer formed on a transparent substrate in this order is opposed to a liquid crystal recording medium having a transparent electrode and a polymer dispersion type of liquid crystal layer formed on a transparent substrate in this order, and voltage is applied between both the electrodes for exposure to image-carrying light, so that the liquid crystals are oriented to record the information. In this system, the transmittance of the liquid crystal recording medium, the current flowing through the photoelectric sensor or liquid crystal recording medium, the conductivity of the unexposed portion of the photoelectric sensor, and the transmittance of the liquid crystal layer at a dark portion are measured, or the voltage applied on the liquid crystal recording medium is estimated by monitoring the current flowing through the liquid crystal recording medium, so that the applied voltage and the duration of the applied voltage can be controlled. Before recording the information, the optimal applied voltage and the optimal duration of the applied voltage can thus be preset to make it possible to obtain images of high quality.

BACKGROUND OF THE INVENTION

This invention relates to a method and device for recording informationon an information recording medium built up of a photoelectric sensorand a liquid crystal layer at a preset optimum applied voltage, and amethod for measuring the transmittance of a liquid crystal recordingmedium which is applied to recording information.

So far, an arrangement wherein a polymer dispersion type of liquidcrystal recording medium--in which a liquid crystal layer having liquidcrystals dispersed and fixed in resin is formed on an electrode--islocated in opposition to a photoelectric sensor having a photoconductivelayer formed on an electrode layer has been known so as to record imagesby exposure to light at an applied voltage.

A typical construction of an image recorder used with such a polymerdispersion type of liquid crystal recording medium is illustrated inFIG. 1, in which reference numerals 10 and 20 stand for a photoelectricsensor and a liquid crystal recording medium, respectively. In thephotoelectric sensor 10 a transparent electrode 12 and a photoconductivelayer 13 are successively coated or otherwise formed on a transparentsupporting substrate 11, and in the liquid crystal recording medium 20 atransparent electrode 22 and a polymer dispersion type of liquid crystallayer 23 are successively formed on a transparent supporting substrate21. The photoconductive layer 13 used, for instance, may have asingle-layer structure in which trinitrofluorenone is added to amorphousselenium or amorphous silicon for an inorganic photoconductive layer orto polyvinylcarbazole for an organic photoconductive layer, a formedstructure in which a carrier generation layer obtained by dispersing anazo type pigment in a resin such as polyvinyl butyral is formed on acarrier transport layer obtained by mixing a hydrazone derivative with aresin such as polycarbonate, or other structures.

An arrangement in which, as shown in FIG. 1, a photoelectric sensor isopposed to a liquid crystal recording medium for exposure to light at anapplied voltage, while a gap of about 10 μm is maintained between themby a spacer of polyethylene, polyimide or other polymer, and anarrangement in which, as shown in FIGS. 2(a) and 2(b), a photoelectricsensor and a liquid crystal recording medium are stacked together, havebeen put forward in the art. The stacked form of recording medium isbroken down into two types, one in which a liquid crystal recordinglayer is stacked directly on a photoelectric sensor, as shown in FIG.2(a), and the other in which an middle layer 24 of a transparentdielectric material is interposed between them, as shown in FIG. 2(b).

When such an arrangement built up of the photoelectric sensor 10 and theliquid crystal recording medium 20 located in opposition thereto isirradiated with visible write light, while voltage is applied betweenboth the electrodes 12 and 22 by a power source 30, the conductivity ofthe photoconductive layer 13 changes depending on the intensity of thelight, resulting in a change in the electric field applied on the liquidcrystal layer 23 and so a change in the orientation of the liquidcrystal layer. This orientation, even when the applied voltage is putoff for removal of the electric field, is maintained so that images canbe recorded.

The thus recorded image information can be read by irradiating theliquid crystal recording medium 20 with read light from a light source40 and reading the transmitted light with a photoelectric conversionelement 60 to convert it to electrical signals, as shown in FIG. 4 byway of example. For the light source 40 a white light source such as axenon or halogen lamp or a laser light source may be used. For the readlight with which the liquid crystal recording medium is irradiated, itis preferable to select light of suitable wavelength by a filter 50. Theincident light is modulated by the orientation of the liquid crystallayer of the liquid crystal recording medium, and the transmitted lightis converted to electrical signals through the photoelectric conversionelement 60 made up of diodes, etc. The resulting electrical signals maybe processed by a printer or otherwise displayed on a CRT, if required.

The orientation of the liquid crystal layer of a liquid crystalrecording medium depends on the magnitude of an applied voltage, theduration of an applied voltage, etc., and for recording images of a highcontrast it is required to measure the degree of orientation of theliquid crystal layer. The degree of orientation of the liquid crystallayer may be found by monitoring the transmittance.

One typical method for measuring the transmittance is illustrated inFIG. 5. As illustrated, a photoelectric sensor 10 having a transparentelectrode 12 and a photoconductive layer 13 successively formed on atransparent supporting substrate 11 and a liquid crystal medium 20having a transparent electrode 22 and a liquid crystal layer 23successively formed on a transparent supporting substrate 21 are opposedto each other with an air gap of about 10 μm between them. At the sametime as the supporting substrate of the photoelectric sensor 10 isirradiated with white light, voltage is applied by a power source 30between the electrodes 12 and 22 with such a polarity that the electrode12 becomes positive. As illustrated, black paper 43 is put on thesurface of the supporting substrate of the photoelectric sensor 10, sothat half the photoelectric sensor can be shielded light. Two sets ofinfrared light-emitting LEDs 41 and photoelectric conversion elements 42are located on the side of the supporting substrate of the liquidcrystal medium 20 in such a way that infrared light from the LEDs passesthrough the liquid crystal medium, and is then reflected by the surfaceof the photoconductive layer of the photoelectric sensor so that it canbe incident on the photoelectric conversion elements. One set is locatedat the (light) portion of the photoelectric sensor that is irradiatedwith light and another is positioned at the (dark) portion of thephotoelectric sensor that is shielded from light. As the transmittanceof the liquid crystal medium increases upon the application of voltage,there is an increase in the quantity of the light incident on thephotoelectric conversion elements 42, the output signals of which aremonitored on an oscilloscope 65. A shutter 52 is located between thephotoelectric sensor 10 and the light source. This shutter is insynchronism with the power source, and is preset such that it is putdown simultaneously with the application of voltage and is put up afterthe lapse of 1/30 sec (33 msec). The signals of the photoelectricconversion element are monitored on the oscilloscope simultaneously withthe application of voltage.

However, this conventional method for measuring the transmittance of aliquid crystal recording medium has a grave problem, because this isdesigned to detect the light that passes through the liquid crystalrecording medium, and is reflected by the surface of the photoelectricsensor; no satisfactory detection signal can be obtained, because thesurface reflectivity of the photoelectric sensor is low, and becauseinfrared light is used as monitoring light so as to prevent thesensitization of the photoelectric sensor, and this places somelimitation upon the usable wavelength region.

The magnitude, duration, etc., of the voltage applied on the liquidcrystal layer may be preset by monitoring the transmittance of theliquid crystal layer. A problem with this method, however, is that anadditional function of measuring the transmittance must be added to theinformation recorder, resulting in an increase in recorder size. Anotherproblem is that the transmittance of the liquid crystal recording layer,because of depending largely on the wavelength of the monitoring light,must be corrected.

Still another problem with image recording according to such a method isthat the recorded image becomes too light or too dark due to a slightdifference in the magnitude and duration of an applied voltage, and thisis true of even when the photoelectric sensor and liquid crystalrecording medium having the same characteristics are used. Thus, it isvery difficult to determine the conditions for recording images desiredfor those who record them, because the properties of the image recordedvary considerably depending on the magnitude and duration of an appliedvoltage.

SUMMARY OF THE INVENTION

An object of the invention is to prevent the sensitization of aphotoelectric sensor by monitoring light and to precisely measure thetransmittance of a liquid crystal recording medium.

Another object of the invention is to determine the necessary electricalproperties of a photoelectric sensor and a liquid crystal recordingmedium before recording information, thereby making it possible todetermine the applied voltage best suited for image recording.

Still another object of the invention is to stop the application ofvoltage after the lapse of the optimal duration of the applied voltage,thereby enabling information to be recorded with a high-enough contrast.

A further object of the invention is to monitor the transmittance of aliquid crystal recording medium at a region corresponding to theunexposed portion, so that the voltage can be put off when a giventransmittance is reached, thereby making it possible to control the toneof the image recorded.

A further object of the invention is to provide a method for recordinginformation, in which an image is recorded while the applied voltage andthe duration of the applied voltage best suited for obtaining a goodimage of a high contrast are preset.

More specifically, the invention provides a method for measuring thetransmittance of a liquid crystal recording medium having a transparentelectrode and a polymer dispersion type of liquid crystal layer formedon a transparent substrate in this order by locating the liquid crystalrecording medium in opposition to a photoelectric sensor having atransparent electrode and a photoconductive layer formed on atransparent substrate in this order and applying voltage between boththe electrodes for exposure to image-carrying light, thereby orientingthe liquid crystals to record the image, characterized in that a part ofthe surface of said photoconductive layer or said liquid crystal layeris provided with a reflecting layer, and incident light passing throughsaid liquid crystal recording medium is reflected by said reflectinglayer to detect the reflected light, so that the transmittance of saidliquid crystal recording medium can be detected.

The invention also provides a method for recording image information bylocating a liquid crystal recording medium including on a transparentsubstrate a polymer dispersion type of liquid crystal recording layer inwhich liquid crystals are dispersed and fixed in resin in opposition toa photoelectric sensor having a photoconductive layer formed on atransparent electrode and applying voltage between both the electrodessimultaneously with the exposure of an image to light, thereby recordingsaid image information, characterized in that, before recording saidinformation, voltage is applied on said photoelectric sensor and/or saidliquid crystal recording medium, and the current flowing therethrough ismeasured to preset the optimal applied voltage.

Further, the invention is characterized in that the conductivity of anunexposed region of said photoelectric sensor is measured, and theduration of the voltage applied between said photoelectric sensor andsaid liquid crystal recording medium is predetermined on the basis ofthe obtained measurements.

Still further, the invention is characterized in that the transmittanceof said liquid crystal recording medium is monitored, and the durationof the applied voltage is controlled on the basis of the transmittanceof said liquid crystal recording medium.

Still further, the invention provides a method for recording informationcharacterized by the transmittance of a liquid crystal layer at a darkportion is detected, and the duration of the applied voltage iscontrolled on the basis of the thus detected results, therebycontrolling the density gradation.

Still further, the invention is characterized in that the currentflowing through a liquid crystal recording medium is monitored toestimate the voltage applied on the liquid crystal recording medium, andthe duration of the applied voltage is controlled on the basis of thethus estimated voltage.

Still further, the invention is characterized in that the currentflowing through a liquid crystal recording medium is measured to monitorthe behavior of the liquid crystal layer, so that the duration of theapplied voltage can be controlled.

Still further, the invention provides a method for applying voltageusing a polymer dispersion type of liquid crystal recording medium,characterized in that the current of the dark portion is measured tomonitor the behavior of the liquid crystal recording layer, and themoment the contrast reaches a maximum is detected to put off thevoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the structure of the liquid crystal recordingmedium,

FIGS. 2-A and 2-B are schematics of the structure of the liquid crystalrecording medium,

FIG. 3 is a schematic illustrating how to record an image,

FIG. 4 is a schematic illustrating how to read an image

FIG. 5 is a schematic illustrating a conventional method for measuringthe transmittance of liquid crystals,

FIG. 6 is a schematic illustrating one example of the method formeasuring the transmittance of liquid crystals according to theinvention,

FIG. 7 is a graph representing the change-with-time in transmittance ofthe light and dark portions,

FIG. 8 is a schematic illustrating another example of the method formeasuring the transmittance of liquid crystals according to theinvention,

FIG. 9 is a schematic illustrating one example of the method formeasuring the transmittance of the light portion,

FIG. 10 is a schematic illustrating another example of the method formeasuring the transmittance of the light portion,

FIG. 11 is a graph showing the results of the transmittance measured,

FIG. 12 is a schematic illustrating the method for measuring thebehavior of the liquid crystal recording layer at the exposed andunexposed portions by the transmittance-measuring method according tothe invention,

FIGS. 13(a) and 13(c) are graphs showing the transmittance change of theliquid crystal recording medium at the exposed and unexposed portions atvarying applied voltages of 670 V, 720 V and 770 V,

FIG. 14 is a graph showing the change-with-time of a transmittancedifference between the exposed and unexposed portions,

FIG. 15 is a graph showing the change-with-time in the degree ofmodulation between the exposed and unexposed portions,

FIG. 16 is a schematic showing the method for measuring the resistanceof the liquid crystal recording medium and the conductivity of thephotoelectric sensor,

FIGS. 17(a)-17(b) are graphs showing the measurements of the currentflowing through the liquid crystal recording layer,

FIG. 18 is a schematic of an equivalent circuit for a recorder made upof a photoelectric sensor and a liquid crystal recording medium,

FIG. 19 schematically illustrates the changes in voltage of thephotoelectric sensor and liquid crystal recording layer at the unexposedportions,

FIG. 20 schematically illustrates the change in current of thephotoelectric sensor and liquid crystal recording layer at the unexposedportions,

FIG. 21 is a graph showing the change-with-time in the density of thecurrent flowing through the photoelectric sensor,

FIG. 22 is a graph showing the change-with-time in the voltage appliedon the capacitor and photoelectric sensor,

FIG. 23 is a graph showing the relation between the voltage and currentof the photoelectric sensor,

FIG. 24 is a schematic illustrating how to measure the characteristicsof the photoelectric sensor,

FIG. 25 is a graph showing the measurements of the current of thephotoelectric sensor,

FIG. 26 is a graph showing the calculated and measured values for thecurrent of the photoelectric sensor,

FIG. 27 is a graph showing one example of the calculations of thevoltage applied on the liquid crystal medium at the light and darkportions,

FIG. 28 is a graph showing one example of the calculations of thedifference in the voltage applied on the liquid crystal medium betweenthe light and dark portions.

FIG. 29 is a schematic showing an equivalent circuit in the case ofusing an integral type of liquid crystal recording medium,

FIG. 30 is a schematic illustrating how to preset the optimal appliedvoltage and the optimal duration of the applied time,

FIG. 31 is a graph showing the modulation-change-with-time of the liquidcrystal recording medium,

FIG. 32 is a graph showing the contrast change with time,

FIG. 33 is a graph showing the change-with-time of the voltage appliedon the liquid crystal recording medium and the capacity change of theliquid crystal recording layer as well,

FIG. 34 is a graph showing the transmittance change of the liquidcrystal recording medium versus the change-with-time of the voltageapplied on the liquid crystal recording medium,

FIGS. 35(a)-35(b) are graphs showing the relation between the lightexposure and the transmittance of the liquid crystal recording mediumwhen a gray scale is subjected to projection exposure,

FIG. 36 is a graph showing the relation between the change in thevoltage applied on the liquid crystal medium and the drive range of theliquid crystals,

FIGS. 37(a)-37(b) are schematics showing how to measure thetransmittance of the unexposed portion,

FIGS. 38(a)-38(b) are schematics illustrating how to measure thetransmitted light,

FIG. 39 is a schematic illustrating how to measure the current of aseparation type of liquid crystal medium,

FIG. 40 is a schematic illustrating how to measure the current of aseparation type of liquid crystal medium,

FIG. 41 is a schematic illustrating how to measure the current of anintegral type recording medium,

FIG. 42 is a schematic illustrating how to measure the current of anintegral type recording medium,

FIG. 43 is a graph showing the relation between the applied on thephotoelectric sensor and the dark current value at that time,

FIG. 44 is a graph showing the results of simulation of the time changesof the voltages applied on the photoelectric sensor and liquid crystallayer,

FIG. 45 is a graph showing the calculations of the current value,

FIG. 46 is a schematic illustrating how to measure the current of theunexposed portion,

FIGS. 47(a)-47(b) are schematics illustrating how to measure thecurrents of the exposed and unexposed portions at the same time,

FIGS. 48(a)-48(b) are schematics illustrating an example wherein thecurrent is measured while a portion provided with a spacer is separatedfrom a portion provided with an electrode,

FIG. 49 is a schematic illustrating how to measure the electricalproperties of the liquid crystal recording layer,

FIG. 50 is a schematic illustrating how to measure the capacity changeand current value of the liquid crystal recording layer,

FIG. 51 is a graph showing the capacity change of the liquid crystalrecording layer,

FIG. 52 is a graph showing the measurements of the current value,

FIG. 53 is a graph showing the change-with-time of the current in thedark portion by the differentiation of the measurements with respect totime,

FIG. 54 is a schematic illustrating how to measure the current of theexposed portion and the modulation of the liquid crystal medium,

FIGS. 55(a)-55(b) are graphs showing the current characteristics of theexposed and unexposed portions,

FIGS. 56(a)-56(b) are graphs showing the capacity changes of the exposedand unexposed portions,

FIG. 57 is a graph showing the differential value of the currentdifference between the exposed and unexposed portions,

FIG. 58 is a graph showing the change-with-time of differentiation ofthe current value of the exposed portion,

FIGS. 59(a)-59(b) are graphs showing the calculations of the voltageapplied on the liquid crystal recording layer and the change-with-timeof the potential difference of the unexposed portion,

FIG. 60 is a schematic illustrating how to measure the capacity andtransmittance changes of the liquid crystal recording layer,

FIG. 61 is a graph showing the measurements of the current valueobtained when a slope form of voltage is applied, and

FIG. 62 is a schematic showing the waveform of the signal from themonitored photoelectric element.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 6 illustrates a typical example of the method for measuring thetransmittance of a liquid crystal medium according to the invention.

A mask 3 is located on the side of a photoelectric sensor 10 to beexposed to image-carrying light, so that this region can be shieldedfrom the light. A reflecting layer 1 made up of a dielectric mirrorlayer or A1, by way of example, is formed on the surface of aphotoconductive layer 13 in opposition to the shielding mask 3, and areflecting layer 2 is formed on the surface of the photoconductive layer13 that is to be exposed to the image-carrying light. The portion of themember 10 provided with the reflecting layer 1 is a dark portion that isshielded by the mask 3 from light and is not exposed to light, where theconductivity of the photoconductive layer 13 is low, while the portionof the member 10 provided with the reflecting layer 2 is a light portionthat is exposed to light, where the conductivity of the photoconductivelayer 13 is high. The voltage applied on the liquid crystal medium ishigher at the light portion than at the dark portion, so that the liquidcrystals can be oriented faster at the light portion than at the darkportion, resulting in an increase in transmittance.

A transmittance sensor 4 is made up of a visible LED and a photoelectricconversion element. When the transmittance of the dark portion ismeasured, the reflecting layer 1 is irradiated with light through aliquid crystal recording medium 20 to detect the light reflected by thefilm 1. When the transmittance of the light portion is measured, thereflecting layer 2 is irradiated with light to detect the lightreflected by the film 2. Since the light reflected by the reflectinglayers on the surface of the photoelectric sensor is detected, thephotoelectric sensor 10 is most unlikely to be sensitized, even whenvisible light is used. By use of the reflecting layers it is possible todetect the reflected light at a very high reflectivity.

FIG. 8 illustrates another example of the method for measuring thetransmittance of a liquid crystal medium according to the invention,wherein only a portion of a photoelectric sensor 10 provided with areflecting layer 5 is exposed to image-carrying light, and no mask isused. The transmittance of the light portion can be monitored bydetecting the light reflected by the reflecting layer 5 with the use ofa transmittance sensor 4.

FIG. 9 illustrates another example of the method for monitoring thetransmittance of the light portion. A photoelectric sensor 10 is exposedto image-carrying light through an image formation lens system 70 and ahalf mirror 53, while an optical shutter 50 is put down for a giventime. Then, the half mirror 53 is used to remove a part of the light,with which a portion of the photoelectric sensor provided with areflecting layer is irradiated through a reflecting mirror 54 and anoptical system 71. Then, the reflecting layer is irradiated through aliquid crystal recording medium 20 with monitoring light from an LED 41.The transmittance of the light portion can be monitored by detecting thelight reflected by the reflecting layer with the use of a photoelectricconversion element 42.

FIG. 10 illustrates still another example of the method for monitoringthe transmittance of the light portion. In this case no half mirror isused; with the use of an optical shutter 51 and an optical system 71 theportion--to be monitored--of a photoelectric sensor provided with areflecting layer is irradiated with a suitable quantity of light.

The quantity of the light incident on the portion to be monitored may beregulated by the lens system 71. While the invention has been describedwith reference to some specific arrangements wherein the reflectinglayers are formed on the surface of the photoelectric sensor, it isunderstood that the invention is not limited to such arrangements.According to the invention, for instance, the monitoring of trasmittancecan be achieved with reflecting layers that are formed on the surface ofthe liquid crystal layer opposite to the photoelectric sensor.

EXAMPLES OF MEASURING TRANSMITTANCE

(a) Fabrication of photoelectric sensor

Three (3) parts of a carrier generation substance, i.e., a fluorenoneazo pigment having the following structural formula and 1 part ofpolyester resin were mixed with 196 parts of a solvent mixtureconsisting of dioxane and cyclohexane at 1:1, and the mixture was wellkneaded together by means of a mixer to prepare a coating solution.##STR1##

This solution was coated on the surface of an ITO transparent electrode(having a thickness of about 500 Å and a resistance value of 80 Ω/) on aglass substrate, and was then dried at 100° C. for 1 hour to form acarrier generation layer having a thickness of 0.3 μm.

Next, 3 parts of a carrier transport substance, i.e., p-dimethylstilbenehaving the following structural formula and 1 part of polystyrene resinwere mixed with, and dissolved in, 170 parts of a solvent mixtureconsisting of dichloromethane and 1,1,2-trichloroethane at 68:102 toprepare a coating solution. ##STR2##

This solution was coated on the above carrier generation layer, and wasthen dried at 80° C. for 2 hour to form a carrier transport layer havinga thickness of 10 μm.

Then, an A1 deposited mask was formed on the surface of the glasssubstrate. Following this, A1 was vapor-deposited on the masked surfaceof the glass substrate at a thickness of about 1,000 Å to form areflecting layer of 5 mm×5 mm in size, thereby fabricating aphotoelectric sensor.

(b) Fabrication of Liquid Crystal Recording Medium

A liquid crystal was extracted from the cross-section of the liquidcrystal recording medium using methanol. After it was dried, it wasexamined under scanning electron microscope (Hitachi; S-800;x 10,000).As a result, it was found that the surface of the layer was covered witha UV-setting resin of 0.6 μm thick, and resin particles with particlesize of 0.1 μm were filled inside the layer. Because the liquid crystalrecording medium has a skin layer comprising UV-setting resin on thesurface of the layer, it has excellent property to retain the liquidcrystal in the medium. Also, because no effusion phenomenon of liquidcrystal occurs on the surface, no disturbance of information recordingoccurs due to the liquid crystal on the medium surface when it is usedas an information recording medium, and it is possible to recordinformation without unevenness.

Preparation of integrated type information recording medium

On the electric charge transport layer of the optical sensor as alreadydescribed, disparaxylilene having the following structure was addedunder vacuum condition by vapor polymerization. Thus, a film ofpoly-(monochloro-paraxylilene) of 0.6 μm thick was formed as adielectric intermediate layer. Further, on the dielectric intermediatelayer, the solution used in the preparation of the information recordingmedium was coated by the same procedure. Through UV-setting, theinformation recording layer was produced. On this information recordinglayer, ITO was laminated by the sputtering method to thickness of about200 Angstrom. Thus, the integrated information recording medium of thepresent invention was prepared. ##STR3##

A mixture of 4 parts of dipentaerythritol hexacrylate, 6 parts of asmectic liquid crystal (S6; Merck), 0.2 parts of a fluorine typeactivator (Fluorad FC-430; Sumitomo 3M) and 0.2 parts of aphotopolymerization initiator (Durocure 1173; Merck) was regulated to asolid content of 30% with xylene.

This solution was coated on the surface of an ITO transparent electrode(having a thickness of 500 Å and a resistance value of 80 Ω/) on a glasssubstrate by means of a blade coater having a gap thickness of 50 μm,and was held at 50° C. and then irradiated with UV light of 0.3 J/cm² tofabricate a liquid crystal recording medium having a liquid crystalrecording layer of about 6 μm in thickness.

(c) Measurements

The thus fabricated photoelectric sensor and liquid crystal recordingmedium were opposed to each other with an air gap of about 10 μm in sucha manner that light from an LED was reflected by the reflecting layer onthe photoelectric sensor, and struck on the photoelectric sensor. Whilea voltage of about 750 V was applied on this arrangement for 200 msec ina dark place, signals from the photoelectric sensor were monitored usingan infrared light-emitting LED (700 nm) and a visible light-emitting LED(500 nm). As a result, waveforms A and B shown in FIG. 11 were obtained,respectively.

FIG. 12 illustrates the method for measuring the behavior of the exposedand unexposed portions of a liquid crystal recording layer, using thetransmittance measuring method mentioned above. A photoelectric sensor10 is opposed to a liquid crystal recording medium 20 with an air gap ofabout 10 μm between them. Using a light source 40 and an optical shutter50, the transparent substrate of the photoelectric sensor is irradiatedwith a given quantity of light for a given time. Half the transparentsubstrate 11 of the photoelectric sensor 10 is provided with a blackmask 14 so as to shield the region of the photoelectric sensorcorresponding to it from light. The photoelectric sensor 10 is providedon the photoconductive layer with reflecting mirror layers 15 and 16.The reflecting mirror layers are separately formed by the vapordeposition of A1, and the reflecting mirror layer 15 is located inopposition to the mask 14. The liquid crystal recording medium 20 isprovided with photoelectric conversion elements 51 and 53 and LEDs 52and 54, so that light beams from the LEDs 52 and 54 can be reflected bythe reflecting mirrors 15 and 16, and strike on the photoelectricconversion elements 51 and 53. The photoelectric sensor is irradiatedwith light from the light source 40 by putting down the optical shutter50, simultaneously with the application of voltage between electrodes 12and 22 via a power source 30, whereby the photoelectric conversionelements 51 and 53 can be used to monitor how the transmittance of theexposed and unexposed portions of the liquid crystal recording layerchanges.

Transmittance changes of the exposed and unexposed portions of theliquid crystal recording medium at varying applied voltages of 670 V,720 V and 770 V were monitored by the photoelectric conversion elements51 and 53. The results are shown in FIGS. 13(a) to (c). Also shown inFIG. 14 are changes-with-time of a difference in the monitoring signalsbetween the exposed and unexposed portions. In FIG. 13 Curves L1, L3 andL5 show how the exposed portion of the liquid crystal medium changes andCurves L2, L4 and L6 show how the unexposed portion changes. In FIG. 14Curves M1, M2 and M3 show changes-with-time of signal differencesbetween the exposed and unexposed portions at the respective voltages.

Since the conductivity of the photoelectric sensor is higher at theexposed portion than at the unexposed portion, an extra voltage isapplied on the liquid crystal recording layer, so that the liquidcrystal layer can be rapidly oriented, resulting in a transmittanceincrease. Thus, images can be recorded due to a difference in the speedof the transmittance change. From FIGS. 13(a) to 13(c) it is alsounderstood that the higher the applied voltage, the higher the voltageand rate of operation of the liquid crystal recording layer. Thedifference in transmittance between the exposed and unexposed portionsreaches a maximum at a certain time; that is, images can be recorded bystopping the application of voltage at that time. As can be seen fromFIG. 14, the time at which the difference in transmittance between theexposed and unexposed portions of the liquid crystal recording mediumreaches a maximum is t1, t2 and t3 at the applied voltages of 670 V, 720V and 770 V; that is, the higher the voltage, the shorter the durationof the applied voltage. The maximum value of the difference intransmittance between the exposed and unexposed portions depends onvoltage; in other words, no enhanced contrast is obtained at too high orlow an applied voltage. To obtain recorded images of good quality, it isthus required to apply the optimal voltage for recording images.

FIG. 15 with time as abscissa and the degree of modulation as ordinateshows the degree of modulation of the exposed (light) and unexposed(dark) portions. The "contrast" is given by a difference in the degreeof modulation between the light and dark portions, which enables imagesto be recorded.

The rate of operation of the liquid crystal is higher at the exposedportion than at the unexposed portion, because the conductivity of thephotoconductive layer of the photoelectric sensor is higher at theexposed portion than at the unexposed portion, resulting in theapplication of an extra voltage on the liquid crystal portion. Hence,images can be recorded by making use of the difference in the rate ofoperation.

For instance, now let us consider the case where the voltage is put offat a certain time. At that moment the operation of the liquid crystalrecording medium stops, so that image information can be recorded. Ascan be seen from FIG. 15, there is the optimal time for putting off thevoltage. For instance, if the voltage is put off too early, e.g., at t1,no sufficient contrast will then be obtained due to no sufficientmodulation of the light portion of the liquid crystal recording medium.If the voltage is applied too long, e.g., for as long as t3, on theother hand, neither sufficient contrast nor any image of good qualitywill again be obtained due to some excessive modulation of the darkportion of the liquid crystal recording medium. To obtain images of goodquality, it is thus required to determine the optimal duration (t2) ofan applied voltage. However, much difficulty is involved in its precisepredetermination before the application of voltage.

Various methods for presetting the voltage to be applied and theduration of an applied voltage according to the invention will now beexplained at great length.

In the first place, reference will be made to the determination of thevoltage to be applied by measuring the resistivity of a liquid crystalrecording layer and the conductivity of a photoelectric sensor.

RESISTIVITY OF LIQUID CRYSTAL RECORDING MEDIUM

As shown in FIG. 16, an Au electrode 17 was deposited on the surface ofa portion separate from the portion of the liquid crystal recordinglayer fabricated as mentioned above, which is to be formed with images.The Au electrode 17 had an area of 0.16 cm², and the liquid crystalrecording portion with the electrode formed on it was found to have anelectrostatic capacity of 150 pF, as measured by an impedance meter.Then, a capacitor 82 of 500 pF was connected in series with the liquidcrystal recording layer, as shown in FIG. 16, to find the current value,while a voltage of 100 V was applied thereon for about 0.1 sac via apower source 31. The current value was measured by measuring the voltageof a resistance 81 of 50 kΩ connected in series with the recordinglayer. The results are shown in FIGS. 17(a) and (b). FIG. 17(a) shows acurrent change from just after the application of the voltage to theputting-off of the voltage, and FIG. 17(b) is a logarithmicrepresentation of a current while voltage is applied.

The liquid crystal recording layer is considered to be a parallelcircuit with the capacitor and resistance. Just after the application ofthe voltage, the voltage is distributed to the liquid crystal recordinglayer and capacitor according to their capacity ratio, and the voltageof the liquid crystal recording layer then decreases due to the flow ofa current through the resistance component of the liquid crystalrecording layer. At the same time as the voltage is put off, a currentof the opposite polarity flows due to the discharge of the capacitor, asshown in FIG. 17(a). Plotting the logarithm of the current versus timegives a straight line such as one shown in FIG. 17(b), so that theresistivity in Ωcm of the liquid crystal recording layer can becalculated from the gradient of that straight line. By calculation, theresistivity of the liquid crystal recording layer fabricated asmentioned above was thus found to be 2.0×10¹¹ Ωcm.

In this invention, the area of the Au electrode and the capacity of thecapacitor connected to it are in no sense limited to the valuesmentioned above. Illustratively and preferably, an electrode of about0.1 to 1 cm² in area is used with a capacitor of 100 to 1,000 pF. Whilethe applied voltage is not particularly limited to 100 V, it is neededto apply a voltage lower than the threshold value. This is because whenthe voltage of the liquid crystal recording layer exceeds the thresholdvoltage, there are changes in the capacity and the current to bemeasured due to the orientation of the liquid crystal, and so no preciseresistivity value is obtained. Also how to measure the resistivity isnever limited to the method mentioned above. For instance, theresistivity may be found by applying a slope form of voltage on theliquid crystal recording layer with no capacitor connected to it andcalculating the gradient of the current value measured.

HOW TO PRESET THE VOLTAGE TO BE APPLIED

In what follows, how to preset the voltage to be applied will beexplained.

In the image recorder system according to the invention, thephotoelectric sensor and the liquid crystal recording medium are eachconsidered to be a parallel circuit with the resistance and capacitor,and represented in terms of an equivalent circuit made up of theseseries circuits, as shown in FIG. 18.

As illustrated, now let C_(S) and C_(L) denote the capacities of thephotoelectric sensor and liquid crystal recording medium, V_(AP)(=source voltage E-gap voltage V_(AIR)) denote a voltage across theseries circuit, V_(S) and V_(C) denote voltages on the photoelectricsensor and liquid crystal recording medium, and I_(S) and I_(L) denotecurrents through the photoelectric sensor and liquid crystal recordingmedium. Just after the application of voltage, the applied voltage isdistributed according to the capacity ratio. Then, the voltages aregiven by

    V.sub.L (0)=V.sub.AP ×C.sub.S /(C.sub.L +C.sub.S)    . . . (1-1)

    V.sub.S (0)=V.sub.AP ×C.sub.L /(C.sub.L +C.sub.S)    . . . (1-2)

Thereafter, the voltage of the liquid crystal recording mediumincreases, because the current I_(S) through the photoelectric sensor islarger than the current I_(L) (=V_(L) /R_(L)) through the liquid crystalrecording medium so that charges can be accumulated in C_(L). This statetakes the form of the following differential equation (1-3): ##EQU1##where dV_(S) /dt=-dV_(L) /dt

FIGS. 19 and 20 are schematic representations of changes in the voltageand current of the photoelectric sensor and liquid crystal layer at theunexposed portion in the case of the present recording system. In thepresent system an image of an enhanced contrast can be obtained bystopping the application of voltage when the voltage of the unexposedportion of the liquid crystal recording layer reaches the thresholdvoltage at which the liquid crystal starts to operate (orient). At thistime the voltage of the liquid crystal recording layer is so equal tothe threshold voltage, so that the current can be given by

    I.sub.L =V.sub.TH /R.sub.L                                 . . . (1-4)

As explained with reference to FIG. 17, the resistivity of the liquidcrystal recording layer may be found from a current change, and becausethe threshold voltage of the liquid crystal recording medium is alreadyknown, the current value of the liquid crystal recording layer may thenbe found by Equation (1-4). By calculation, the current value of theliquid crystal recording medium fabricated as mentioned above was foundto be 1.5×10⁻⁶ A/cm².

When the preset applied voltage is too low, no image recording canoccur, because the voltage of the liquid crystal recording layer cannotexceed the threshold voltage. At too high a preset applied voltage thevoltage of the liquid crystal recording medium reaches the thresholdvoltage too early, and so any image of an enhanced contrast cannot beobtained due to a small difference in the degree of orientation betweenthe unexposed and exposed portions. Such a high preset applied voltageis thus not preferable. When the voltage of the liquid crystal recordinglayer is the threshold voltage, the current through the photoelectricsensor must have a proper value preset on the basis of the current thatthen flows through the liquid crystal recording layer. Thus, an image ofan enhanced contrast can be obtained by presetting the applied voltagein such a way that the current, which flows through the photoelectricsensor when the voltage of the liquid crystal recording layer is thethreshold voltage, has a proper value, as will be described at greatlength.

From Equation (1-3), it is understood that when the liquid crystalrecording layer starts to operate upon its voltage reaching thethreshold voltage, there is a current corresponding to an increase inthe electrostatic capacity of the liquid crystal recording layer due tothe orientation of the liquid crystal. When the application of thevoltage is stopped, the voltage change of the liquid crystal recordinglayer is considered to be small (i.e., dV_(L) /dt≈0 in Equation (1-3)),and from Equation (1-3) it is then considered that the proper currentvalue of the photoelectric sensor is the sum of the current flowing fromthe resistance component of the liquid crystal recording layer and theportion corresponding to the capacity change. The current density of theliquid crystal recording layer prepared as mentioned above thatcorresponds to the capacity change is 1 to 3×10⁻⁶ A/cm², preferablyabout 2×10⁻⁶ A/cm². This value remains substantially unchanged even whenthere is a variation in the thickness of the liquid crystal recordinglayer. In some cases, however, it varies depending on the liquid crystalsubstance used and a change in its composition with resin. In otherwords, the current density of the liquid crystal recording layer must beestimated with the rate of the capacity change of the liquid crystalrecording medium in mind. The current corresponding to the voltagechange of the liquid crystal recording layer and photoelectric sensor issmaller than the capacity variable components and so is negligible,because the voltage change is not appreciably large, as alreadymentioned.

MEASUREMENT OF THE CONDUCTIVITY OF PHOTOELECTRIC SENSOR

The conductivity of the photoelectric sensor may be measured, as shownin FIG. 16.

An Au electrode 18 of 0.16 cm² in size is deposited on the surface ofthe photoconductive layer at a portion of the photoelectric sensor thatis not to be formed with an image (or that is provided with the mask14). The photoelectric sensor was found to have a capacity of 50 pF, asmeasured with an impedance meter. As in the case of the liquid crystalrecording layer, a capacitor 84 of a suitable capacity was connected inseries with the photoelectric sensor to apply voltage across it via apower source 32, thereby measuring the current. The current was measuredby measuring the voltage of a resistance 83 of 50 kΩ. Unlike the currentthrough the liquid crystal recording layer, the current through thephotoelectric sensor is not proportional to voltage; that is, no linearrelation is obtained, even if the logarithm of the current value isplotted versus time. For this reason, the measurement of theconductivity of the photoelectric sensor and the presetting of thevoltage to be applied must be done as follows.

Just after the application of voltage, the voltage is distributed to thephotoelectric sensor and capacitor according to their capacity ration.Here let V_(AP), C and V_(C) denote an applied voltage, the capacity ofthe capacitor 84 and the voltage on it. Then, the voltages are given by

    V.sub.C (0)=V.sub.AP ×C.sub.S /(C+C.sub.S)           . . . (2-1)

    V.sub.S (0)=V.sub.AP ×C/(C+C.sub.S)                  . . . (2-2)

Thereafter, the voltage of the photoelectric sensor decrease with time,because a current I_(S) flows through the photoelectric sensor. Thisstate takes the form of the following differential equation (2-3):

    I.sub.EX =C(dV.sub.C /dt)=I.sub.S +C.sub.S (dV.sub.S /dt)  . . . (2-3)

Here,

    dV.sub.C /dt=-dV.sub.S /dt                                 . . . (2-4)

so that ##EQU2## where I_(EX) is the measured current.

Thus, the current value and, hence, the conductivity of thephotoelectric sensor can be found from the measured current.

Shown in FIG. 21 is the current through the photoelectric sensoraccording to the present embodiment, which was measured at an appliedvoltage of 300 V with a capacitor of 200 pF capacity.

The voltage distributed to the capacitor just after the application ofthe voltage is given by Equation (2-1). Thereafter, the voltage of thecapacitor changes with time, and this can be calculated from ##EQU3##

Also the change-with-time of the voltage on the photoelectric sensor canbe calculated from the following equation (2-7):

    V.sub.AP =V.sub.C +V.sub.S. . . (2-7)

From the results of the current measured the change-with-time of thevoltages on the capacitor and photoelectric sensor can be found, asshown in FIG. 22, and the relation between the voltage and current ofthe photoelectric sensor is shown in FIG. 23.

PRESETTING OF APPLIED VOLTAGE

Reference will now be made to how to preset the voltage to be applied onthe basis of the results of the resistivity of the liquid crystalrecording medium measured and the results of the current of thephotoelectric sensor measured.

When the application of voltage is stopped, the voltage of the liquidcrystal recording medium is tantamount to the threshold voltage, asalready noted. It is then considered that the liquid crystal recordinglayer undergoes no substantial voltage change, and that the currentthrough the liquid crystal recording layer is given by the sum ofcurrent components due to the resistance and capacity variablecomponents in Equation (1-3). The current through the resistancecomponent is given by Equation (1-4), and the liquid crystal recordingmedium prepared as mentioned above is found to be 1.5×10⁻⁶ A/cm². If thecurrent due to the capacity variable component is 2.0×10⁻⁶ A/cm², thecurrent through the photoelectric sensor must then be 3.5×10⁻⁶ A/cm².

FIG. 21 shows that just when 26 msec elapses after the initiation of theapplication of voltage, the current of the photoelectric sensor has theabove value. The voltage that is then applied on the photoelectricsensor can be calculated by integrating the measured current from timet=0 to t=t_(e) according to Equations (2-6) and (2-7). In actual imagerecording, it is thus possible to find the voltages of the photoelectricsensor and liquid crystal recording layer at the time when theapplication of voltage is stopped. In other words, the applied voltageto be preset can be found by adding the discharge voltage of the air gapto these voltages. That is,

    V.sub.AP =V.sub.S +V.sub.TH +V.sub.AIR                     . . . (2-8)

Here

V_(AP) =the applied voltage V_(S) =145 V

V_(TH) =the threshold voltage of the V_(TH) =180 V liquid crystalrecording layer

V_(AIR) =the discharge voltage of the V_(AI) =400 V/725 V air gap

From Equation (2-8) and the results measured, it was found that at athreshold voltage of 180 V the optimal applied voltage was 725 V for thephotoelectric sensor and liquid crystal recording medium fabricated asmentioned above.

In the following description, the characteristics of the photoelectricsensor used in the invention will be explained.

FIG. 24 illustrates how to measure the characteristics of thephotoelectric sensor used in the invention. A gold electrode 14 isdeposited on a photoconductive layer 13 of a photoelectric sensor 10over an area of 0.16 cm². Through a transparent substrate 11, the golddeposited portion of the photoelectric sensor 10 is irradiated withlight from a light source 51. A shutter 52 located between the lightsource 51 and the photoelectric sensor 10 can be used to achieve a giventime of irradiation of the photoelectric sensor with light. A suitableresistance (50 kΩ) and a power source 30 are connected in series betweena transparent electrode 12 and the gold electrode 14, and a constantvoltage is applied therebetween via the power source 30. After about 200msec elapsed from the initiation of application of voltage, the shutter52 was opened up for 33 msec. The current value of the photoelectricsensor was then measured by detecting the voltage on the resistanceconnected in series with the photoelectric sensor.

One example of the results measured is shown in FIG. 25. Here thecurrent value found in the absence of light is called a dark current anda difference between a current in the presence of light and the darkcurrent a photo-current. The photoelectric sensor used in the inventionis of an injection type. According to this type photoelectric sensor,the photo-current increases while irradiated with light, so that thesensitivity can be enhanced due to this amplifying effect. Even afterthe irradiation of the photoelectric sensor with light is stopped (attime t₁), the photo-current attenuates gently. Thus, the duration of thephoto-current is enough-long as long as the voltage is applied.

No precise clarification of the mechanism of this photoelectric sensorhas been made as yet, because of its very complicated behavior. In theinvention, however, it is found that the behavior of this photoelectricsensor can be expressed in terms of a function of voltage and time. Thisis because, as a result of studies made of the dependence of thecharacteristics of the photoelectric sensor upon voltage, the behaviorof the photoelectric sensor can be approximated as follows.

(1) The dark current is proportional to the square of the voltage of thephotoelectric sensor. That is,

    I.sub.d =αV.sup.2                                    . . . (3-1)

where α is a constant and V is the voltage applied on the photoelectricsensor.

(2) The photo-current is broken down into that obtained while theirradiation of the photoelectric sensor with light is held on and thatobtained after the irradiation of the photoelectric sensor with light isput off. (2-1)

When the voltage is kept constant, the photo-current (a differencebetween the portions of the photoelectric sensor that are irradiated,and not irradiated, with light) increases with time while thephotoelectric sensor is irradiated with light, as can be seen from FIG.25. It is difficult to express a photo-current change in terms of asimple formula, but the following linear approximate expression can holdfor a region exposed to light of low intensity (of up to 50 LUX) for ashort time (of up to 100 msec). That is,

    ΔI(t)=kt                                             . . . (3-2)

where ΔI(t) is the photo-current and k is a constant. (2-2)

When the voltage is kept constant, the photo-current attenuates at acertain time constant after the irradiation of the photoelectric sensorwith light is put off. That is, the following approximate expression canhold.

    ΔI(t)=kt.sub.1 exp{(t.sub.1 -t)/τ}               . . . (3-3)

where t₁ is the time at which the irradiation of the photoelectricsensor with light is put off, and τ is a time constant (200 to 500msec).

(3) As a result of studies made of the dependence of the photo-currentupon voltage, the photo-current is found to be proportional to voltage,when the electric field strength is in a certain range (of 5 to 49V/μm). From (1) to (3), it is found that the current of thephotoelectric sensor that is being irradiated with light is a functionof the voltage of the photoelectric sensor and time with respect to acertain light intensity, and is given by

    I.sub.p (V.sub.p, t)≈αV.sub.p.sup.2 +βV.sub.p t (0<t≦t.sub.1)                                      . . . (3-4)

    I.sub.p (V.sub.p, t)≈αV.sub.p.sup.2 +βV.sub.p t.sub.1 exp{(t.sub.1 -t)/τ}(t.sub.1 <t)                       . . . (3-5)

where V_(p) is the voltage of the photoelectric sensor, t₁ is the timeat which the irradiation of the photoelectric sensor with light is putoff, and β is a constant.

The values obtained by calculation from Equations (3-4) and (3-5) andthe found values are shown in FIG. 26, wherein characteristic curves Aand B represent the calculated and found values, respectively.

In the ensuing description, reference will be made to how to calculatethe voltage applied on the liquid crystal medium 10 for recording imageswith the use of a separation type of liquid crystal recording mediumsuch as one shown in FIG. 3.

In an image recorder built up of such a separation type of liquidcrystal recording medium, it is considered that the breakdown voltage ofair discharge is applied on the air layer portion, and there is anequivalent circuit such as one shown in FIG. 18.

Just after the initiation of the application of voltage, it isconsidered that the voltage is distributed to the photoelectric sensorand liquid crystal medium according to their capacity ratio. Thevoltages are then given by

    V.sub.S (0)={C.sub.L /(C.sub.S +C.sub.L)}×(E-Va)     . . . (4-1)

    V.sub.L (0)={C.sub.S /(C.sub.S +C.sub.L)}×(E-Va)     . . . (4-2)

Also the following differential equations hold.

    C.sub.S (dV.sub.S /dt)+I.sub.S =C.sub.L (dV.sub.L /dt)+I.sub.L. . . (4-3)

    V.sub.S +V.sub.L =(E-Va)                                   . . . (4-4)

The differentiation of both sides of Equation (4-4) with respect to timebecomes 0. From this and Equation (4-3),

    dV.sub.L /dt=(I.sub.S -I.sub.L)/(C.sub.S +C.sub.L)         . . . (4-5)

so that,

    V.sub.L (t+Δt)=V.sub.L (t)+(dV.sub.L /dt).Δt   . . . (4-6)

From this approximate expression the voltage imposed on the liquidcrystal medium was calculated.

Here, if the liquid crystal medium 10 is taken as being a parallelcircuit with the capacitor and resistance, the current flowing throughthe liquid crystal medium can then be found by

    I.sub.L =V.sub.L /R                                        . . . (4-7)

The current of the photoelectric sensor varies at the light and darkportions, and are given by the following equations.

The current of the dark portion is given by

    I.sub.S (d)=αV.sub.S (d).sup.2                       . . . (4-8)

The current of the light portion is given by

    I.sub.S (p)=αV.sub.S (p).sup.2 +βV.sub.S (p).t  . . . (4-9)

on condition that 0<t≦t₁, and by

    I.sub.S (p)=αV.sub.S (p).sup.2 +βV.sub.S (p).t.sub.1 exp{(t.sub.1 -t)/τ}                                   . . . (4-10)

on condition that t1<t.

Here α is a constant that can be determined by measuring the currentvalue of the dark portion of the photoelectric sensor and found by

    I.sub.S (m)=αV.sub.S (m).sup.2                       . . . (4-11)

where I_(S) (m) is the value measured and V_(S) (m) is the voltagemeasured.

Also β is found from α by calculating from the following equation:

    βV.sub.S (m)t.sub.1 /αV.sub.S (m).sup.2 =1.25   . . . (4-12)

As can be seen from Equation (4-9), the numerator and denominator ofEquation (4-12) stand for the photo-current and dark current at time t₁,respectively, and this ratio may lie in the range of about 1.10 to about1.50. In the invention, however, the ratio is preferably 1.25. Thevoltages on the liquid crystal medium at the light and dark portions canbe calculated by substituting Equations (4-7) to (4-10) for Equation(4-6). Some results are shown in FIGS. 27 and 28.

Now consider the condition under which the difference in the voltageapplied on the liquid crystal medium between the light portion and thedark portion reaches a maximum. As can be seen from FIG. 27, the riseand saturation of the voltage of the light portion occur earlier than dothose of the dark portion. That is, that condition is considered to holdwhen the rate of the change of the voltage applied on the liquid crystalmedium is invariable at the light and dark portions or is larger at thedark portion than at the light portion. From a comparison of the rightside of Equation (4-5)--that represents the rate of the change in thevoltage applied on the liquid crystal medium--between the light portionand the dark portion, it is understood that the condition under whichthe voltage difference reaches a maximum is given by the followingrelation:

    I.sub.S (d)-I.sub.L (d)≧I.sub.S (p)-I.sub.L (p)     . . . (4-13)

The first moment the condition of Relation (4-13) is met is consideredto be the time at which the difference in the voltage applied on theliquid crystal medium between the light portion and the dark portionreaches a maximum. Thus, the voltage to be applied is preferably presetsuch that the voltage of the dark portion is equal to, or slightlylarger (e.g., by a few volts) than, the threshold of the liquid crystalmedium. It is here to be noted that the optimal time of the applicationof voltage is the time at which the difference in the voltage applied onthe liquid crystal medium between the light portion and the dark portionreaches a maximum.

More specifically, the voltage applied on the liquid crystal medium at acertain preset voltage is first calculated from Equation (4-6). Then,the voltage to be applied is calculated from Relation (4-13). However,when the voltage of the liquid crystal medium at the dark portion islarger than the threshold of the liquid crystal medium, the appliedvoltage is reset low for re-calculation. When the voltage of the liquidcrystal medium at the dark portion is smaller than the threshold of theliquid crystal medium, on the other hand, the applied voltage is resethigh for recalculation. Thus, the optimal value for the applied voltagecan be found by calculation such that the voltage of the dark portion isequal to, or slightly larger (e.g., by a few volts) than, the thresholdof the liquid crystal medium.

Next, reference will be made to how to calculate the voltage to beapplied on an integral type of liquid crystal medium such as one shownin FIG. 2.

An integral type medium made up of three layers, i.e., photoelectricsensor, middle and liquid crystal layers, is represented by such anequivalent circuit as shown in FIG. 29. It is here to be noted thatI_(M) and C_(M) represent the resistance and capacity of the middlelayer.

Just after the initiation of application of voltage, the voltage isdistributed to the photoelectric sensor, middle and liquid crystallayers according to their capacity ratio. The voltages are then given by

    V.sub.S (0)=C.sub.M C.sub.L /(C.sub.S C.sub.M +C.sub.M C.sub.L +C.sub.L C.sub.S)×E                                          . . . (5-1)

    V.sub.M (0)=C.sub.L C.sub.S /(C.sub.S C.sub.M +C.sub.M C.sub.L +C.sub.L C.sub.S)×E                                          . . . (5-2)

    V.sub.L (0)=C.sub.S C.sub.M /(C.sub.S C.sub.M +C.sub.M C.sub.L +C.sub.L C.sub.S)×E                                          . . . (5-3)

From the equivalent circuit shown in FIG. 28, the following differentialequation holds: ##EQU4##

If Equation (5-4) is solved, then the voltages applied on the middlelayer and crystal liquid medium can be calculated from ##EQU5##

Equations (4-7) to (4-10) and (5-8) are substituted for Equation (5-7)to calculate the voltages applied on the light and dark portion of theliquid crystal.

As in the case of the separation type of liquid crystal medium, thecondition under which the difference in the voltage applied on theliquid crystal medium between the light portion and the dark portion isthe time at which the rate of the change in the voltage applied on theliquid crystal medium is always the same at the light and dark portions,or is larger at the dark portion than at the light portion. From acomparison of the right side of Equation (5-7) between the light portionand the dark portion,

    I.sub.S(d) -(I.sub.L(d) -I.sub.M(d))(C.sub.S /C.sub.M)-I.sub.L(d) ≧I.sub.S(p) -(I.sub.L(p) -I.sub.M(p))(C.sub.S /C.sub.M)-I.sub.L(p). . . (5-9)

The first moment the condition of Relation (5-9) is met is considered tobe the time at which the difference in the voltage applied on the liquidcrystal medium between the light portion and the dark portion reaches amaximum. Thus, the voltage to be applied is preferably preset such thatthe voltage of the dark portion is equal to, or slightly larger (e.g.,by a few volts) than, the threshold of the liquid crystal medium. It ishere to be noted that the optimal time of application of voltage is thetime at which the difference in the voltage applied on the liquidcrystal medium between the light portion and the dark portion reaches amaximum.

More specifically, the voltage applied on the liquid crystal medium at acertain preset voltage is first calculated from Equations (5-6) and(5-7). Then, the voltage to be applied is calculated from Relation(5-9). However, when the voltage of the liquid crystal medium at thedark portion is larger than the threshold of the liquid crystal medium,the applied voltage is reset low for re-calculation. When the voltage ofthe liquid crystal medium at the dark portion is smaller than thethreshold of the liquid crystal medium, on the other hand, the appliedvoltage is reset high for re-calculation. Thus, the optimal value forthe applied voltage can be found by calculation such that the voltage ofthe dark portion is equal to, or slightly larger (e.g., by a few volts)than, the threshold of the liquid crystal medium.

FIG. 30 is a schematic for illustrating how to preset the optimalvoltage to be applied and the optimal duration of the applied voltage.

As in the case of conventional recording methods, a photoelectric sensor10 is opposed to a polymer dispersion type of liquid crystal medium 20with a gap between them. However, a part of the carrier transport layerof the photoelectric sensor used in the present embodiment is providedthereon with an electrode 14. The liquid crystal medium 20 is designedto be smaller than, or be displaced with respect to, the photoelectricsensor 10, so that nothing is located in front of the portion of thephotoelectric sensor 10 that is provided with the electrode 14. Asillustrated, a mask 43 is provided on a portion of the substrate of thephotoelectric sensor on the opposite side of the electrode 14, wherebythat portion is not exposed to light. A power source 31 is used to applya rectangular voltage between an electrode 12 of the photoelectricsensor and the electrode 14 on a photoconductive layer, so that acurrent flowing through the photoelectric sensor can be determined bymeasuring the voltage of a resistance connected in series with it. Sincethe area of the electrode formed on the photoconductive layer is alreadyknown, α can be found from the current value measured by Equation (3-1).

Then, β is found by Equation (4-12), so that the optimal values for thevoltage to be applied and the duration of the applied voltage can bedetermined according to the method expressed by Equations (4-1) to(4-13). In connection with Equation (4-12), the ratio of the darkcurrent to the photo-current has been prima facie described as being1.25, but it is understood that this is not particularly limited to1.25, because there is no appreciable variation in calculations in therange of 1.10 to 1.50.

The duration of the applied voltage may be determined either by thevalue found by such calculation or by monitoring the transmittance ofthe liquid crystal medium at the dark portion as usual. Here it is notedthat the actually applied voltage must be found by adding the dischargebreakdown voltage to the calculated value, because no gap voltage isincluded in the voltage found by calculation. The discharge breakdownvoltage shall follow Paschen's law.

As in the case of an integral type of liquid crystal medium, anelectrode is provided on a part of the carrier transport layer of thephotoconductive layer, but neither middle layer nor liquid crystal layeris formed on the electrode. The current is measured as in the case of aseparation type medium, and the optimal value of the voltage to beapplied can then be calculated from Equations (5-1) to (5-7). In thiscase, no care may be taken of the discharge breakdown voltage due theabsence of any gap.

Reference will now be made to an embodiment wherein the transmittance ofthe dark or light portion is measured to detect the moment the contrastreaches a maximum, thereby putting the applied voltage off.

The contrast (quality) of an image changes with time, and is enhanced atan applied voltage duration of 55 to 75 msec, so that good-enough imagequality can be obtained. For instance, when the voltage is put off at anapplied voltage duration of about 30 msec, no good-enough image qualitycan be obtained, because no appreciable change in the liquid crystals atthe light portion results in a low contrast. When the voltage is put offat an applied voltage duration of about 120 msec, no good-enough imagecan again be obtained due to an increase in the transmittance of thedark portion.

Thus, the contrast of an image varies largely depending on the appliedvoltage duration; it is required to control the applied voltage durationprecisely. However, it is difficult to make an accurate prediction ofthe optimal value of the applied voltage duration before an image ispicked up, because it varies depending on the characteristics of thephotoconductive and liquid crystal recording layers and the environmentin which an image is picked up. It is thus required to incorporate afunction of controlling the applied voltage duration in an imagerecorder.

A change-with-time of the modulation of a liquid crystal recordingmedium is illustrated in FIG. 31 with M1, M2 and M3 denoting the degreesof modulation of the light portion, the middle portion (that is an areairradiated with light corresponding to the average quantity of the lightused for exposure) and the dark portion, respectively. The contrast isgiven in terms of a difference between characteristic curves M1 and M3,and the contrast characteristics are shown in FIG. 32. Now let T_(ON),T_(OFF) and T denote the maximal transmittance (of the liquid crystalsin a completely oriented state), the transmittance of the liquidcrystals when they are not oriented at all and the transmittance of theliquid crystals in a certain oriented state, respectively. Then, thedegree of modulation M is defined by

    M=(T-T.sub.OFF)/(T.sub.ON -T.sub.OFF)

From FIG. 32 it is noted that the contrast peaks in the vicinity of 0.07sec.

As can be seen from FIG. 31, the contrast (quality) of an imagecorrelates with the degree of modulation of the light, middle or darkportion. For instance, the degree of modulation is of the order of 90%at the light portion (Level L1), about 50% at the middle portion (LevelL2), and about 20% at the dark portion (Level L3). First, thetransmittance of the liquid crystal medium at the light, middle, dark,or other region exposed to a suitable quantity of light is monitored todetermine the transmittance at which the contrast reaches a maximum asby the method explained with reference to FIG. 5. Then, the applicationof voltage is stopped when the transmittances of the light, middle anddark portions of the liquid crystal medium reach a level at which thecontrast peaks. In the example illustrated, the application of voltageis stopped, when the transmittances of the light, middle and darkportions reach Levels L1, L2 and L3. By this it is possible to obtainthe optimal applied voltage duration at which the contrast peaks and soan image of good quality is achievable. Hence, it is also possible toachieve density gradation control by varying the applied voltageduration on the basis of the transmittance of the dark portion, forinstance.

Of course, it is also possible to stop the application of voltage bymonitoring the transmittances of the light and dark portions, findingthe transmittance difference to detect the contrast, and detecting themoment the contrast peaks. The moment the contrast peaks may be detectedat the timing at which the contrast change is minimized.

In connection with the measurement of the transmittance of the liquidcrystal recording medium explained with reference to FIG. 5, it is notedthat the reflectivity of the photoelectric sensor is low. To obtainsufficient signals, it is thus required to increase the intensity of theLED. In this case, however, the photoelectric sensor is irradiated withsome considerable light. To reduce the light--with which thephotoelectric sensor is irradiated--as much as possible, it ispreferable that an LED having a wavelength lying in the infraredregion--to which the photoelectric sensor has a relatively lowsensitivity--is used at a light emitting cycle of 0.9 msec with respectto a 0.1 msec exposure to light.

In connection with the method shown in FIG. 5, it is noted that thetransmittance of the liquid crystal medium does not substantially changewith respect to the light in the infrared region; it is difficult todetect the light by the photoelectric sensor. The method for irradiatingthe photoelectric sensor with periodically pulsed light is also notpreferable, because the light is poor in stability and makes the circuitcomplicated as well. Therefore, the transmittance of the liquid crystalmedium may be measured by the methods explained with reference to FIGS.8 to 10.

Reference will now be made to another embodiment in which thetransmittance of the liquid crystal recording medium is monitored torecord an image.

In the instant embodiment, the transmittance of a portion of the liquidcrystal medium corresponding to an unexposed portion of thephotoelectric sensor is monitored. When the transmittance of the liquidcrystal recording medium changes to a given level, the applied voltageis put off, thereby controlling the tone of the image to be recorded.

FIG. 33 shows the calculations of the change-with-time of the voltageapplied on the liquid crystal recording medium, when the photoelectricsensor is unexposed with light according to the present method ofrecording information. However, it is understood that the photoelectricsensor and liquid crystal recording medium are calculated as being aparallel circuit with a resistance and a capacitor. From FIG. 33, it isfound that the initial voltage corresponding to the capacity ratio ofthe liquid crystal recording medium and the photoelectric sensor isimposed on the liquid crystal recording medium, but the voltage thenincreases with time.

FIG. 34 shows a change in the transmittance of a liquid crystalrecording medium with respect to a change-with-time in the voltage ofthe liquid crystal recording medium. With the voltage imposed on theliquid crystal recording medium exceeding the threshold of the liquidcrystal recording medium, the liquid crystals are so oriented that thetransmittance of the liquid crystal recording medium can increase. Shownin FIG. 34 is the transmittance change of the liquid crystal recordingmedium with respect to light of 365 nm wavelength, although it variesdepending on wavelength. It is understood that the transmittance of theliquid crystal recording medium is expressed in terms of a relativevalue with respect to a 100% transmittance that is obtained when theliquid crystals are completely oriented.

Shown in FIG. 35 is the relation between the light exposure and thetransmittance of a liquid crystal recording medium with respect to lightof 365 nm, when a gray scale is subjected to projection exposure tolight, with FIG. 35(a) represented in terms of logarithm and FIG. 35(b)on an isometric scale. The light exposure is a relative value, and thetransmittance of the liquid crystal recording medium is again expressedin terms of a relative value with respect to a 100% transmittance thatis obtained when the liquid crystals are completely oriented. With thesame photoelectric sensor and liquid crystal recording medium asmentioned above, recording was made under the following two conditionsfor the application of voltage:

Black Dots: 780 V 80 msec

White Dots: 740 V 80 msec

Thus, a change in the voltage-applying condition results in a change inthe voltage imposed on the liquid crystal recording medium even at theunexposed portion and so a change in how an image is recorded at theunexposed portion. As illustrated, there is a difference in how torecord an image between when the transmittance (a portion shown bybroken lines) of the unexposed portion is about 10% (white dots) andwhen the transmittance of the unexposed portion is about 20% (blackdots). The reproducibility of the area exposed to a low quantity oflight is better when the transmittance of the unexposed portion is 20%than when the transmittance of the unexposed portion is 10%. When thetransmittance of the unexposed portion is 10%, the area, which can beexpressed when the transmittance of the unexposed portion is 20%, isflat. At the area exposed to a high quantity of light, no informationcan be expressed even by liquid crystals that are not saturated when thetransmittance of the unexposed portion is 10%. The reason is that whenthe transmittance of the unexposed portion is 20%, nearly 100% of theinformation is transmitted.

Thus, there is a variation in the exposed area expressed by the imagerecorded depending on the transmittance change of the unexposed portion.This relation results from the characteristics of the liquid crystalrecording medium, and holds even when the photoelectric sensor used incombination with the liquid crystal recording medium varies.

By making use of such properties, it is possible to enhance the areaexposed to a low quantity of light, as shown in FIG. 36 by way ofexample. This is achieved by putting off the voltage after thetransmittance of the unexposed portion increases to some extent (timet3). Likewise, the area exposed to a high quantity of light can beenhanced by putting off the voltage when the transmittance of theunexposed portion changes only a little (time t1). Thus, an image of therequired tone can be recorded.

It is thus possible to control the properties of the image recorded bymonitoring the transmittance of the unexposed portion. In addition, itis possible to record various images enhanced at the area exposed to alow or high quantity of light.

For a conventional recording medium, it is required to keep the quantityof light exposed thereon constant by controlling the incident light bymeans of a diaphragm incorporated in the input system or changing theexposure time depending on a change in the area of the image (to berecorded) exposed to light. As can be seen from FIG. 35, however, it ispossible to record an image varying in the area exposed to light byvarying the transmittance of the unexposed portion. This is because whenthe image is recorded under such conditions that the transmittance ofthe unexposed portion is high, the area exposed to light shifts to thearea exposed to a low quantity of light.

The recorded image, because of being sufficiently reduced in terms ofnoise, can be converted by a reader to electrical signals having thedesired properties.

Reference will now be made to how to measure the transmittance of theunexposed portion with reference to FIG. 37.

As shown in FIG. 37(a), a photoelectric sensor 10 located at theposition where the transmittance of a liquid crystal recording medium 20is to be measured is provided on the surface with a mask 6 whichprevents any irradiation of the photoelectric sensor with light. In thisarrangement, light from an LED 41 passes through the liquid crystalmedium 20, is reflected by the surface of a photoconductive layer 13 ofthe photoelectric sensor 10, and strikes on a photodiode 42. As shown inFIG. 37(b), a very thin reflecting layer 7 such as a dielectric mirrorlayer may be formed on the surface of the photoconductive layer. It isunderstood that

when no reflecting layer is provided, some care must be taken to preventany sensitization of the photoelectric sensor by the light from the LED.Then, a power source circuit is regulated such that when the signal ofthe photodiode has a given value, i.e., the transmittance of theunexposed portion reaches a given level, the applied voltage is put off.

Read light for the liquid crystal recording medium is different inwavelength from the light generated from the LED. In addition, a readerreads the transmitted light and the light from the LED reads thereflected light. As a result, the degree of scattering of thetransmitted and reflected light in the liquid crystal recording mediumdiffers, resulting in a change in the behavior of the transmittancechange. Thus, both the transmitted light and the reflected light must becorrelated with each other for correction.

FIG. 38(a) shows how to measure the transmitted light. A liquid crystalrecording medium is provided on a glass substrate including an ITCtransparent electrode, and an ITO electrode 24 is formed on the surfaceof the liquid crystal layer by means of sputtering. Light from a lightsource 41 is filtered through a filter 81 to obtain only a lightcomponent of 365 nm, with which the portion of the liquid crystal layerprovided with the ITO electrode is irradiated through an aperture 82.Then, the transmitted light is monitored by a photodiode 42. A powersource 3 is used to apply between both electrodes of the liquid crystalrecording medium a voltage that increases on a constant slope. Then, thesignal and current value of the photodiode are monitored on anoscilloscope 90. It is not always necessary to limit the transmittedlight to 365 nm, but it is required to regulate the wavelength and theoptical system as well in association with how to read.

FIG. 38(b) shows how to measure monitor signals. A liquid crystalrecording medium is provided thereon with an A1 electrode 25. In thisarrangement, light from an LED 41 is reflected by the A1 electrode, andstrikes on a photodiode 42. Likewise, a power source 3 is used to applyon the recording medium a slope form of voltage to monitor the signalsand current value of the photodiode on an oscilloscope 90. By comparingthese measurements, it is possible to examine the correlation of achange in the transmittance of the read system with a transmittancechange in image recording.

Reference will now be made to an embodiment wherein the current flowingthrough the dark portion is measured to detect the moment the contrastpeaks, thereby putting the voltage off.

A liquid crystal recording medium has an intrinsic threshold voltage.Upon exceeding the threshold voltage, the liquid crystals in the liquidcrystal recording layer line up in the direction of an applied electricfield, causing the liquid crystal recording layer to turn from opaque totransparent. The rate of modulation of the liquid crystal recordinglayer in the vicinity of the threshold voltage is already known or, ifnot so, can be measured just before recording information. Thus, if thetime at which the voltage imposed on the liquid crystal recording layerbecomes the threshold voltage, it is then possible to determine the timeat which a transmittance of 10 to 20% is obtained by adding the timerequired for modulation to that time.

A liquid crystal recording layer is considered to be a parallel circuitwith a resistance and a capacitor. Thus, the current flowing through itcan be measured at the time of recording information to monitor thevoltage applied on the liquid crystal recording layer, as will bedescribed later, thereby finding the time at which the voltage on theliquid crystal recording layer becomes the threshold voltage.

First, how to measure the current will now be explained.

How to measure the current in a separation type of information recordingmedium is illustrated in FIG. 39, wherein an electrode of aphotoelectric sensor is shown to be separated into two regions 12a and12b. In other words, voltage can independently be applied on theelectrode 12a that is an image-recording portion and the electrode 12bthat is a current-measuring portion. The supporting substrate of thephotoelectric sensor 10 is provided on the surface with a mask 15 thatcan cover all the photoconductive layer formed on the current-measuringelectrode 12b. The mask may be formed by the deposition of a reflectinglayer such as an A1 layer, the coating of black ink, or the attachmentof an opaque seal. In addition, a part of the surface of thephotoconductive layer shielded from light is provided with an electrode14. The electrode 14 is not always required to be transparent, and somay be made up of an ITO electrode, a deposited A1 layer, etc. Also theelectrodes 12b and 14 are not always required to have the same area. Byway of example but not by way of limitation, it is desired that bothelectrodes overlap over an area lying in the range of 0.1 to 1 cm².

A part of the surface of the liquid crystal recording layer is providedwith an electrode 25. A power source 30c can be used to apply voltage onthat electrode to measure the current through it, thereby determiningthe electrical properties (resistance and electrostatic capacity) of theliquid crystal recording layer.

The resistance and capacity of the liquid crystal recording layer arecalculated as the area of the portions of the electrodes 12b and 14 thatoverlap each other. A variable resistance 41 and a capacity 42 areregulated to the thus calculated value and, as shown, connected inseries with the photoelectric sensor together with a current-measuringelectrode 40a, followed by the application of voltage via a power source30b. Although varying depending on the area of the electrode, thecurrent-measuring resistance 40a used may have a resistance value of 1kΩ to 100 MΩ, and has preferably a resistance value variable dependingon the resistance of the liquid crystal recording medium and the area ofthe electrode. The voltage to be applied is determined by subtractingthe discharge voltage of an air layer from the voltage of theimage-recording power source 30a. Here, too, the discharge voltage shallPaschen's law. Consequently, the current measured by thecurrent-measuring resistance 40a can be taken as being a current flowingthrough the liquid crystal recording medium.

The current may be measured by the simultaneous application of voltagefrom the power sources 30a and 30b simultaneously with image recording.Alternatively, voltage may be applied from the power source 30b alone tothe recording medium just before image recording to calculate theapplied voltage duration, followed by image recording with the powersource 30a. The method of measuring the current simultaneously withimage recording is more advantageous, because the time of imagerecording can be made shorter. However, the method of measuring thecurrent just before image recording has a merit of being able to correctthe applied voltage, when it is considerably different from the propervalue. Of course, these methods may be used in combination with eachother.

In the above methods of measuring the current, it is required toseparate the electrode of the photoelectric sensor into thecurrent-measuring and image-recording portions. As shown in FIG. 40,however, it is also preferable to apply a bias voltage 31 correspondingto the air discharge voltage to a current-measuring circuit and, at thesame time, apply voltage thereto via a common power source 30, whereby avoltage except the discharge voltage is applied on the current-measuringportion. In this case, the element (resistance and capacity) of a pseudoliquid crystal recording layer is determined by the area of an electrode14.

Reference will now be made to how to measure the current of an integraltype recording medium in which photoconductive, dielectric middle andliquid crystal recording medium are formed on a transparent electrode inthis order.

This method is illustrated in FIGS. 41 and 42 wherein, as in the case ofthe arrangement shown in FIG. 39, an electrode is shown to be separatedinto an image-recording portion and a current-measuring portion. Asupporting substrate is provided on the surface with a light-blockingmask 15. In the arrangement shown in FIG. 41, a photoconductive layer isprovided thereon with an electrode 14, and a resistance 41 and acapacitor 42 that correspond to a liquid crystal recording layer areconnected in series with a resistance 43 and a capacitor 44corresponding to a dielectric middle layer. The resistor and capacity ofthe liquid crystal recording layer are measured in the same manner asillustrated in FIG. 39, and are connected with each other with anelement calculated as the area of the current-measuring portion. Theresistance and capacity of the dielectric middle layer may be estimatedfrom the dielectric constant, resistivity, thickness and area thereof. Apower source 30b is used to apply voltage on a current-measuringresistance 40 connected to it, thereby measuring the current and socalculating the applied voltage duration.

In the arrangement shown in FIG. 42, a dielectric middle layer isprovided on the surface with an electrode 14, which is connected with aresistance 41 and a capacitor 42 corresponding to a liquid crystalrecording layer. By the application of voltage via a power source 30b itis likewise possible to measure the current and so calculate the appliedvoltage duration.

Reference will now be made to how to calculate the applied voltageduration.

A liquid crystal recording layer is considered to be a parallel circuitwith a resistance and a capacitor. After the voltage is initiallydistributed to a photoelectric sensor and a liquid crystal recordingmedium according to their capacity ratio, the following differentialequations hold. Consequently, the voltages can successively becalculated by measuring the current value.

Such a system is represented by an equivalent circuit such as one shownin FIG. 18.

In the initial stage of the application of voltage, the voltage isdistributed to the photoelectric sensor and liquid crystal recordingmedium according to their capacity ratio.

    V.sub.S (0)=V.sub.AP ×(C.sub.L /(C.sub.S +C.sub.L))  . . . (6-1)

    V.sub.L (0)=V.sub.AP ×(C.sub.S /(C.sub.S +C.sub.L))  . . . (6-2)

The liquid crystal recording layer is represented in terms of a parallelcircuit with a resistance and a capacitor, and the followingdifferential equations hold therefor.

    I=C.sub.L (dV.sub.L /dt)+(V.sub.L /R.sub.L)                . . . (6-3)

    V.sub.L (t+Δt)=V.sub.L (t)+(dV.sub.L /dt).Δt   . . . (6-4)

From Equations (6-3) and (6-4),

    V.sub.L (t+Δt)=V.sub.L (t)+(I(t)-V.sub.L (t)/R.sub.L)Δt/C.sub.L. . . (6-5)

By substituting the initial condition of Equation (6-2) for Equation(6-5) it is possible to find the voltage imposed on the liquid crystalrecording layer.

An integral type medium made up of three layers, i.e., a photoconductivelayer, a dielectric middle layer and a liquid crystal recording layer isrepresented by an equivalent circuit such as one shown in FIG. 28.

Just after the initiation of the application of voltage, the voltage isdistributed to the photoconductive, dielectric middle and liquid crystalrecording layers according to their capacity ratio. The distributedvoltages are then given by

    V.sub.S (0)=C.sub.M C.sub.L /(C.sub.S C.sub.M +C.sub.M C.sub.L +C.sub.L C.sub.S)×V.sub.AP                                   . . . (7-1)

    V.sub.M (0)=C.sub.L C.sub.S /(C.sub.S C.sub.M +C.sub.M C.sub.L +C.sub.L C.sub.S)×V.sub.AP                                   . . . (7-2)

    V.sub.L (0)=C.sub.S C.sub.M /(C.sub.S C.sub.M +C.sub.M C.sub.L +C.sub.L C.sub.S)×V.sub.AP                                   . . . (7-3)

Since the three layers are connected in series with one another,Equation (6-3) holds for the liquid crystal recording layer. As in thecase of the separation type, it is thus possible to calculate thevoltage from Equation (6-5) and so find the initial condition fromEquation (7-3).

When the capacity of the photoelectric sensor is smaller than that ofthe liquid crystal recording medium, the voltage imposed on thephotoelectric sensor can be estimated from the current value.

Shown in FIG. 43 is the relation between the voltage imposed on thephotoelectric sensor and the then dark current value. Thus, the voltagecorrelates with the dark current; that is, the voltage can be estimatedfrom the current value. As in the case of Equation (6-3), the followingdifferential equation holds for the photoelectric sensor, when thevoltage is applied on it.

    I=C.sub.S dV.sub.S /dt+I.sub.S (V.sub.S)                   . . . (8-1)

In the information recording system according to the invention, thevoltage imposed on the liquid crystal recording layer drops sharply justafter the application of voltage, but the voltage hardly changes whenthe application of the voltage is stopped. Thus, the current inassociation with the voltage change given by the first term of Equation(8-1) decreases with the duration of the applied voltage.

FIG. 44 shows the results of simulation of changes-with-time in thevoltages applied on the photoelectric sensor and liquid crystal layerwhen the liquid crystal recording medium is provided in the form of aparallel circuit with a capacitor and a resistance. L1 and L2 representthe changes-with-time in the voltages applied on the photoelectricsensor and liquid crystal layer, respectively. As can be seen from FIG.44, they are charged in a short time (of about 2 msec) to the voltagescorresponding to the capacity ratio of the photoelectric sensor to theliquid crystal recording layer. After this, the voltage of thephotoelectric sensor decreases with an increase in the voltage of theliquid crystal recording layer. The rate of change is large at theinitial time of the application of voltage, and decreases with the lapseof time.

The above simulation was done on condition that the liquid crystalrecording layer is parallel with the resistance and capacitor and thedark current of the photoelectric sensor is proportional to the squareof the voltage. The electrical properties of the photoelectric sensorand liquid crystal recording medium used are mentioned below.

Thickness of Recording Medium: 6 μm

Capacity of Recording Medium: 1,000 pF/cm²

Resistivity of Recording Medium: 1.3×10¹¹ Ωcm

Thickness of Photoelectric sensor: 10 μm

Capacity of Photoelectric sensor: 300 pF/cm²

Dark Current of Photoelectric sensor: 1.0×10⁻⁶ A/cm² at an appliedvoltage of 100 V

Applied Voltage: 340 V (except that of the air gap portion)

The results of the calculated current value are also shown in FIG. 45.

In FIG. 45 L3 denotes the change-with-time in the dark current value ofthe photoelectric sensor (the second term of Equation (8-1)) and L4stands for the change-with-time in the current measured (the totalcurrent value I of Equation (8-1)).

As shown in FIG. 43, the voltage and dark current of the photoelectricsensor have a mutual relation, and so the voltage applied on thephotoelectric sensor can be found from the dark current value. What ismeasured by the methods shown in FIGS. 39 and 41 is the sum of theportions corresponding to the dark current and voltage change of thephotoelectric sensor, as can be understood from Equation (8-1). In otherwords, it is impossible to directly measure the dark current value. Ascan be seen from FIG. 45, there is no large difference between thecurrent value L4 measured and the dark current value L3 of thephotoelectric sensor because of the small capacity of the photoelectricsensor. In particular, since there is no appreciable voltage change whenthe voltage of the liquid crystal recording layer reaches the thresholdvoltage, the first term of Equation (8-1) is sufficiently smaller thanthe second term; that is, the current measured can be deemed as beingthe dark current of the photoelectric sensor.

Consequently, when the relation between the dark current and voltage ofthe photoelectric sensor is previously known, it is possible to estimatethe voltage applied on the photoelectric sensor by monitoring thecurrent value of the dark portion, and it is then possible to find thevoltage of the liquid crystal recording layer by subtracting theestimated value from the applied voltage.

Reference will now be made to how to determine when the application ofvoltage is stopped.

As already noted, the current may be measured either at the same as theexposure with an applied voltage or by applying voltage on thecurrent-measuring portion alone just before information recording.

As already mentioned, the condition for the application ofvoltage--under which an image having a high contrast is obtained--isthat the transmittance of the liquid crystal recording medium at thedark portion is about 10 to 20%. Consequently, it is required to findthe optimal duration of the applied voltage by adding the time--at whichthe liquid crystal recording layer is modulated to a transmittance of 10to 20%--to the time at which the voltage of the liquid crystal reachesthe threshold.

The time at which the voltage of the liquid crystal recording layerreaches the threshold can be found by estimating the voltage bymeasuring the current value by the method mentioned above anddetermining whether or not the estimated voltage have reached thethreshold. It is noted that the judgment of whether or not the voltageestimated from the current value measured have reached the thresholdvoltage and the operation for finding the optimal duration of theapplied voltage by the addition of the time at which the liquid crystalrecording layer is modulated to a transmittance of 10 to 20% may becarried out by use of a microcomputer or other control, and the voltageis put off by the microcomputer when the optimal duration of the appliedvoltage is reached. It is also noted that the time necessary for theliquid crystal recording layer to be modulated to a transmittance of 10to 20%, although varying somewhat depending on the liquid crystalmedium, can be previously measured, for instance, by applying a voltagein the vicinity of the threshold voltage to the recording layer andirradiating the recording layer with laser light to measure theintensity of the transmitted light.

Reference will now be made to an embodiment in which the behavior of aliquid crystal recording layer is monitored to detect the moment thecontrast reaches a maximum, thereupon putting off the voltage.

In this embodiment, the behavior of the liquid crystal recording layeris monitored by measuring the current flowing through the liquid crystalrecording medium. For instance, the "behavior" of the liquid crystalrecording layer means that the voltage of the unexposed portion reachesthe threshold voltage--this may be found by measuring the current, thatthe contrast of the light and dark portions reaches a maximum--this maybe found by a change-with-time in the current of the exposed portion ora change-with-time of the difference in the current between the exposedand unexposed portions, and that the difference between the integralvalue of the current of the region irradiated with light and theintegral value of the current of the exposed portion reaches thequantity corresponding to the capacity change of the liquid crystalrecording medium. The moment these are monitored, the application ofvoltage is stopped, so that the image information can be carried outwith the maximum contrast.

First, reference will be made to the method, medium and device formeasuring the current in recording information.

FIG. 46 shows how to measure the current of the unexposed portion. Asillustrated, a part of a transparent supporting substrate of aphotoelectric sensor 10 is provided with a mask 14 to shield aphotoconductive layer at this region from light. The mask 14 may beformed of any desired material with the proviso that it can shield thephotoconductive layer from light, and so may be formed by the coating ofa black coating material or by use of a deposited A1 layer. In thiscase, a transparent electrode of the photoelectric sensor is separatedinto an image information-recording portion and a currentvalue-monitoring portion, as shown at 12a and 12b. While the electrodeof the photoelectric sensor is shown to be separated into 12a and 12b,it is understood that the electrode of the liquid crystal recordingmedium may also be separated into similar portions. While separate powersources are shown to be used with the image information-recording andcurrent value-monitoring portions, it is understood that a single commonpower source may be used. A current-monitoring resistance 40 isconnected between the electrode 12b of the current-monitoring portionand the power source, so that the current can be measured by monitoringthe voltage of the resistance 40. The resistance 40 used may have aresistance value of about 1 kΩ to about 1 MΩ, although varying dependingon the area of the portion to be monitored and the conductivity of thephotoelectric sensor.

FIGS. 47(a) and 47(b) show how to measure the current values of theexposed and unexposed portions at the same time. As shown at 12a, 12band 12c, a transparent electrode of a photoelectric sensor is separatedinto three regions, on which voltage can independently be applied. Theelectrodes 12b and 12c for monitoring the currents of the unexposed andexposed portions are connected with resistances 40b and 40c,respectively, so that the currents can be monitored by measuring thevoltages of the respective resistances. As in the case of thearrangement shown in FIG. 46, a light-shielding mask 14 is formed on thesurface of the supporting substrate of the photoelectric sensor thatcorresponds to the region for monitoring the current of the unexposedportion. It is noted that the current-monitoring region of the exposedportion may be irradiated with light, as shown in FIGS. 9 and 10.

While mention has been made of the method in which the current ismonitored at the current-monitoring region separate from theimage-recording portion, it is understood that the duration of theapplied voltage may also be controlled by monitoring the current of theimage-recording portion. In this case, it is considered that the currentvalue of the image to be recorded at the time when it is irradiated withan average quantity of light is measured. In the method in which thephotoelectric sensor is opposed to the liquid crystal recording mediumwith an air gap between them for the application of voltage, the spacerfilm is located around the image-recording portion to keep the air gapuniform. When the current value of the image-recording portion is found,however, it is impossible to measure the current precisely, if somevoltage is applied on the spacer portion. As shown in FIG. 48 by way ofexample, it is thus required that the portion at which a spacer 15 islocated be separated form the portion at which the electrode 12 islocated. In FIG. 48, the spacer 15 in a rectangular frame form islocated around the photoelectric sensor 10 and liquid crystal recordingmedium 20 such that the image is recorded at the central portion inwhich the spacer 15 does not exist.

When both the current value of the image-recording portion and thecurrent value of the unexposed portion are monitored to control theduration of the applied voltage, it is not always necessary to make theareas of both portions equal to each other. In other words, controloperation can be simplified by use of a resistance in which care istaken of a difference in area.

For instance, let Sa denote the electrode area of the image-recordingportion, Sf represent the electrode area of the current-monitoringportion of the unexposed region and Ra and Rf stand for monitoringresistances. Then,

    SaRa=SfRf

According to this equation resistances are chosen. By comparing thevoltages of the selected resistances it is possible to directly comparethe currents per unit area.

To control the time for putting off the voltage by measuring thecurrent, it is required to measure the electrical properties of a liquidcrystal recording layer. How to measure the electrical properties of theliquid crystal recording layer will now be explained with reference toFIG. 49.

A liquid crystal recording layer is provided on the surface with anelectrode 24, which is connected with a current-measuring resistance 41.A power source 31 is then used to apply on the electrode a pulse voltagelower than the threshold of the liquid crystal recording medium, wherebythe resistance value of the liquid crystal recording medium can be foundby monitoring the current values.

How to control the duration of an applied voltage will now be explainedwith reference to the drawings, using a separation type recording mediumby way of example.

A system of the separation type recording medium is represented by anequivalent circuit such as one shown in FIG. 18. In the initial stage ofthe application of voltage, the voltage is distributed to thephotoelectric sensor and liquid crystal recording medium according totheir capacity ratio.

    V.sub.S (0)=V.sub.AP ×(C.sub.L /(C.sub.S +C.sub.L))  . . . (9-1)

    V.sub.L (0)=V.sub.AP ×(C.sub.S /(C.sub.S +C.sub.L))  . . . (9-2)

The liquid crystal recording layer is represented by a parallel circuitwith a resistance and a capacitor, for which the following differentialequation holds.

    I=d(C.sub.L V.sub.L)/dt+(V.sub.L /R.sub.L)                 . . . (9-3)

With V_(L) lower than the threshold of the liquid crystal recordingmedium, the liquid crystal recording medium does not operate, resultingin no change in the capacity of the liquid crystal recording layer.

    C.sub.L =const                                             . . . (9-4)

    I=C.sub.L (dV.sub.L /dt)+(V.sub.L /R.sub.L)                . . . (9-5)

    V.sub.L (t+Δt)=V.sub.L (t)+(dV.sub.L /dt).Δt   . . . (9-6)

From Equations (9-3) and (9-6),

    V.sub.L (t+Δt)=V.sub.L (t)+(I(t)-V.sub.L (t)/R.sub.L) Δt/C.sub.L. . . (9-7)

Substituting the initial condition of Equation (9-2) for Equation (9-7)enables the voltage applied on the liquid crystal recording layer to befound by calculation.

The capacities of the liquid crystal recording layer and photoconductivelayer can be previously calculated from Equations (9-2) and (9-7), andthe resistance R_(L) of the liquid crystal recording layer can be eithermeasured in advance or measured by the method shown in FIG. 49 justbefore recording an image. At the time of recording the image, thecurrent I(t) of the dark portion can be measured by the method shown inFIG. 46.

To obtain an image of good quality and a high contrast, it is requiredto put off the voltage when the voltage of the liquid crystal recordingmedium reaches the threshold, with the liquid crystals oriented to someextent (about 10 to 20%). The liquid crystals of the dark portion arehardly oriented until the voltage is put off, and so the capacity of theliquid crystal recording layer is considered to remain substantiallyconstant; that is, Equation (9-7) holds for. It is thus possible toobtain an image of a high contrast by putting off the voltage after thevoltage of the liquid crystal recording layer found by measuring thedark current value has reached the threshold voltage. A good image isobtained by putting off the voltage while the liquid crystal layer ofthe dark portion is in a state oriented to some extent (about 10 to 20%of the completely oriented state), rather than in a not-oriented-at-allstate. In other words, it is preferable to put off the voltage after thelapse of some time upon the threshold voltage being reached.

Controlling the duration of the applied voltage may be achieved byanother method in which the capacity change of the liquid crystalrecording layer is detected from the dark current value.

Equation (9-5) holds for the liquid crystal recording layer, because theliquid crystals are not oriented at a voltage lower than the thresholdvoltage, resulting in no change in the capacity of the liquid crystalrecording layer. While voltage is being applied on the liquid crystalrecording layer, the voltage of the liquid crystal recording layerincreases monotonously, with a monotonous decrease in the current valuemeasured. When a voltage higher than the threshold voltage is applied onthe liquid crystal recording layer, the liquid crystals are oriented inthe direction of the electric field, causing a change in the capacity ofthe liquid crystal recording layer. Consequently, Equation (9-3) takesthe following form:

    I=C.sub.L (dV.sub.L /dt)+V.sub.L (dC.sub.L /dt)+(V.sub.L /R.sub.L). . . (9-8)

This means that the capacity change must be taken into consideration.

A change in the capacity of the liquid crystal recording layer, ifoccurs, causes a current to flow according to the second term ofEquation (9-8). Here it is noted that liquid crystals have a dielectricconstant higher in an oriented state than in a non-oriented state.Hence, as the liquid crystals are oriented, there is an increase in thecapacity of the liquid crystal recording layer. By making use of this todetect a change in the current value in association with the capacitychange, it is possible to detect the capacity change (orientation) ofthe liquid crystals.

FIG. 50 is a schematic of how to measure the capacity change and currentvalue of the liquid crystal recording layer according to the invention.

It is required to shield a photoconductive layer of a photoelectricsensor 10 from light either by providing a transparent supportingsubstrate of the photoelectric sensor with a mask 14 or by measuring thecapacity change and current value of a liquid crystal recording layer ina dark environment. A reflecting layer 16 is provided either on thesurface of the liquid crystal recording layer or on the surface of thephoto-conductive layer of the photoelectric sensor. The reflecting layershould be made so extremely thin (1,000 Å or less), thereby preventingthe recording properties of the instant system from being adverselyaffected. As illustrated, the photoelectric sensor is opposed to theliquid crystal recording medium, and an LED 80 and a photoelectricconversion element 81 are positioned such that light from the LED isreflected by the reflecting layer 16, and strikes on the photoelectricconversion element 81. When no reflecting layer is formed on thephotoconductive layer, care must be taken such that the photoelectricsensor is not sensitized by the light from the LED. A power source 30 isused to apply voltage on the photoelectric sensor and the liquid crystalrecording layer, between which an air gap is interposed. The current canbe measured by monitoring the voltage of a resistance 40. As the liquidcrystals are oriented and so the transmittance of the liquid crystallayer changes, the quantity of the light exposed to the photoelectricelement 81 changes. This change can be converted into electrical signalswhich can then be monitored. Shown in FIG. 51 is the capacity change ofthe liquid crystal recording layer measured in this way. The method fordetermining the relation between the signals of the photoelectricconversion element and the capacity of the liquid crystal recordinglayer will later be explained with reference to FIG. 60. Also, thecurrent values measured are shown in FIG. 52. All the values arerewritten per unit area (cm²).

FIG. 53 shows the differential values of the current of the dark portionwith respect to time, i.e., the change-with-time of the current. Whenthe liquid crystals are not oriented at a voltage lower than thethreshold voltage, the current value decreases monotonously, so that thedifferential value is negative. As the liquid crystals start to line up,there is a capacity change. When the quantity of change is large, thesecond term of Equation (9-8) increases with a current increase, so thatthe differential value can be larger than zero.

The liquid crystal layer reaches the threshold voltage in the vicinityof 20 msec, and starts to operate. The quantity of change of the liquidcrystal recording layer increases from the vicinity of 60 msec, and sothe portion corresponding to the capacity change of the second term ofEquation (9-8) can increase. Consequently, the current starts toincrease, so that the differential value can change from negative topositive. Thus, the current value can be monitored to monitor how thecapacity of the liquid crystal recording layer changes on the basis ofthe differential value.

At this time, it is possible to obtain an image of a high contrast bycontrolling the duration of the applied voltage, i.e., putting off thevoltage at the timing at which the current of the dark portion changesfrom the decrease to increase phase (i.e., the differential valuechanges from negative to positive).

Reference will now be made to how to control the duration of the appliedvoltage by measuring the current of the exposed portion.

How to measure the current of the light (exposed) portion and themodulation (capacity change) of the liquid crystal medium is illustratedin FIG. 54. This method is basically similar to that shown in FIG. 50with the exception that the photoelectric sensor is irradiated withlight for a given time, using a light source 50 and an optical shutter51.

Shown in FIGS. 55(a) and 55(b) are the current characteristics A of the(exposed) portion when it is irradiated with light for 1/60 sec, thecurrent characteristics B of the (unexposed) portion when it is notirradiated with light, and the difference (A-B) in the current valuebetween the exposed and unexposed portions. Also shown in FIGS. 56(a)and 56(b) are the modulation (capacity changes C1 and C2 of theunexposed and exposed portions) of the liquid crystal medium, and thecapacity difference (C1-C2) between the unexposed and exposed portions.These values are all rewritten with respect to unit area. Much morecurrent flows through the exposed portion than through the unexposedportion, so that the liquid crystal portion can be modulated faster dueto an excessive voltage applied thereon. As can be seen from FIG. 55(b),the difference in the current value between the exposed and unexposedportions increases from the initiation of the application of voltage tothe completion of irradiation with light (time t1). Even after thecompletion of irradiation with light, the photo current continues toflow, but there is a decrease in the current difference between theexposed and unexposed portions. When the liquid crystal recording layeris thereafter modulated at the exposed portion, a current correspondingto the capacity change represented by the second term of Equation (9-8)flows through the liquid crystal recording layer, so that the currentdifference between the exposed and unexposed portions can again bechanged to the increase phase (time t2). Moreover, when the liquidcrystal layer starts to be modulated at the unexposed portion, thecurrent difference starts to decrease because of the flow of a currentin association with the capacity change (time t3).

The differential values of the current difference between the exposedand unexposed portions are shown in FIG. 57.

From FIG. 56(b), it is found that the difference in modulation (orcapacity) between the exposed and unexposed portion reaches a maximumwhen the voltage is put off in 55 to 70 msec, and this coincides withthe time when the differential value of the current difference shown inFIG. 57 reaches a minimum. FIG. 58 shows the change in differentiationof the current value of the exposed portion with respect to time. Fromthe fact that the minimal value in FIG. 57 coincides nearly with theminimal value in FIG. 58, it is understood that such a change depends onthe operation of the liquid crystal recording layer at the exposedportion. That is, the liquid crystal recording layer at the exposedportion is not constant in terms of the quantity of the modulationchange, and so is rapidly modulated to some extent, but then decreasesin the rate of modulation. This appears to be the reason that suchcurrent characteristics are obtained.

Thus, the voltage can be put off at the timing at which the currentdifference between the exposed and unexposed portions or thedifferential value of the current of the exposed portion assumes aminimal value after the completion of exposure, and so the applicationof voltage can be stopped in a state where the rate of modulation of theexposed portion drops or the liquid crystals are almost completelyoriented, thereby making image recording under the condition under whichan image of a high contrast can be obtained. However, it is then notpreferable to use light having too low an intensity for the purpose ofexposure. It other words, it is preferable to measure the current valuewith respect to light, the intensity of which is at least 80% of thequantity of light corresponding to the maximum light exposure.

Reference will now be made to another method for measuring the currentvalue to control the duration of the applied voltage, i.e., integratingthe difference in the current value between the exposed and unexposedportions.

Shown in FIG. 59(a) are the results of the voltage applied on the liquidcrystal recording layer, which was calculated from Equation (9-8) on thebasis of the measurements shown in FIGS. 55 and 56, and shown in FIG.59(b) is the change-with-time of the potential difference between theexposed and unexposed portions. As can be seen from FIG. 59(b), thepotential difference between the exposed and unexposed portionsincreases initially, but decreases with a change in the capacity of theliquid crystal layer at the exposed portion due to its fastermodulation, and eventually reduces to almost zero in the vicinity of thetime at which the capacity difference between the exposed and unexposedportions reaches a maximum. At this time, the voltage of the liquidcrystal recording layer at the exposed and unexposed portions isapproximately equal to the threshold voltage, and so a difference in thequantity of charges on the liquid crystal recording layer between theexposed and unexposed portions is represented by

    ΔQ=V.sub.TH ΔC                                 . . . (9-9)

where ΔC is the capacity difference between the exposed and unexposedportions, and V_(TH) is the threshold voltage of the liquid crystallayer. Then, the relation between the current values of the exposed andunexposed portions and Equation (9-9) is represented by

    ∫(I.sub.photo -I.sub.dark)dt -∫(ΔV/R.sub.L)dt=ΔQ. . . (9-10)

where R_(L) is the resistance of the liquid crystal recording layer andΔV is the potential difference between the exposed and unexposedportions.

Here the value of ΔQ on the right side of Equation (9-10) can beestimated from Equation (9-9) by measuring the intensity of light. Forinstance, the reason is that when the exposure intensity has a certainvalue, to what degree the exposed and unexposed portions are modulatedcan be presumed and so the threshold voltage can be found. Also, thesecond term of Equation (9-10), viz., a leakage from the resistancecomponent of the liquid crystal recording layer can be pre-estimated.Therefore, if the application of voltage is stopped at the time when thecumulated value (quantity of charges) of the current difference betweenthe exposed and unexposed portions is tantamount to a difference in thequantity of electrification at the time when the capacity differencebetween the exposed and unexposed portions reaches a maximum, it is thenpossible to obtain an image of a high contrast.

In this case, no critical limitation is on the intensity of light forthe exposed portion. For instance, when measuring the current value of aportion irradiated with light in a quantity of 50% of the maximumquantity of light, a suitable reference value may be obtained bymultiplying 50% of the capacity difference between the completelyoriented liquid crystal layer and the non-oriented liquid crystal layerby the threshold value.

The intensity of light for the current-measuring portion need not beuniform all over the region to be measured; only an average quantity oflight need be estimated, because the current value measured is obtainedin the form of an average value. It is also not always required toprovide the current-measuring portion separately from theimage-recording portion. If an average intensity of light can bemeasured, it is then suitable to measure currents all over theimage-recording portion.

Thus, the currents of the exposed and unexposed portions are monitored,so that the behavior of the liquid crystal recording layer can bemonitored, and the voltage is put off after the lapse of a suitableduration of the applied voltage, so that a good image can be obtained.Here, too, the operation of monitoring the behavior of the liquidcrystal recording layer by measuring the current, thereby finding theoptimal duration of the applied voltage, is conducted with the use of amicrocomputer or other control, so that the voltage can be put off bythe microcomputer after the lapse of the optimal duration of the appliedvoltage.

Reference will now be made to how to measure the relation between thecapacity and transmittance changes of the liquid crystal recording layerwith reference to FIG. 60.

A gold electrode 24 is formed on the surface of a liquid crystalrecording layer of a liquid crystal recording medium, and voltage isapplied between the electrode 24 and a transparent electrode 22 via apulse generator (operating as a power source in FIG. 60) and anamplifier. As in the arrangement shown in FIG. 50, an LED 80 and aphotoelectric conversion element 81 are positioned such that, asillustrated, light from the LED is reflected by the gold electrode, andstrikes on the photoelectric conversion element, so enabling atransmittance change of the liquid crystal recording layer to bemonitored.

Applied on the liquid crystal recording layer is a slope form of voltagetaking the following form:

    V(t)=αt                                              . . . (9-11)

As a result of measuring the current value, such a current as shown inFIG. 61 is measured. The liquid crystal recording layer is considered tobe a parallel circuit with a capacity and a resistance, and the currentis given by

    I(t)=C.sub.L (dV.sub.L /dt)+V.sub.L (dC.sub.L /dt)+V.sub.L /R.sub.L. . . (9-12)

From Equation (9-11),

    dV.sub.L /dt=α                                       . . . (9-13)

Also, the resistance R_(L) of the liquid crystal recording layer can beestimated from the gradient of the current, so that the relation betweenthe transmittance and capacity changes can be estimated from acomparison of the results of the current measured with the signals ofthe photoelectric element.

FIG. 62 shows the signals of the thus monitored photoelectric conversionelement. From a comparison of them with the current measured, a peakcurrent is observed at the voltage at which the signals of thephotoelectric element change (corresponding to the transmittancechange). This shows that the transmittance and capacity changes of theliquid crystal recording layer are correlated with each other, and bycomparing both it is possible to find the correlation between thecapacity change and the transmittance-monitoring signals.

What we claim is:
 1. A method for recording information by oppositelyfacing across an air gap a photoelectric sensor having a transparentelectrode and a photoconductive layer formed on a transparent substratein this order to a liquid crystal recording medium having a transparentelectrode and a polymer dispersion type of liquid crystal recordinglayer formed on a transparent substrate in this order, and applyingvoltage between both the electrodes for exposure to image-carryinglight, so that the liquid crystals are oriented to record theinformation, characterized in that the current flowing through saidliquid crystal recording medium is monitored to calculate the voltageapplied on said liquid crystal recording medium, and the duration of theapplied voltage is controlled on the basis of the calculated voltage. 2.A method according to claim 1, characterized in that the duration of theapplied voltage is controlled on the basis of the voltage of theunexposed portion calculated by monitoring the current.
 3. A method forrecording information according to claim 2, wherein voltage applicationis stopped at the time when the voltage on the unexposed portioncalculated by monitoring electric current has reached the thresholdvalue of the liquid crystal recording medium.
 4. A method according toclaim 1, characterized in that an electrode is formed on a part of saidphotoconductive layer, and an electric circuit element corresponding tosaid liquid crystal recording layer is connected to said electrode, sothat when the voltage is applied, the current flowing through saidelectric circuit element is monitored to calculate the voltage appliedon said liquid crystal recording layer, and the duration of the appliedvoltage is controlled on the basis of the calculated voltage.
 5. Amethod for recording information according claim 4, wherein a pole isformed on a part of the photoconductive layer, an electric circuitelement corresponding to the liquid crystal recording layer is connectedto said pole, the electric current flowing through the electric circuitelement when voltage is applied is monitored to calculate the voltageapplied on the liquid crystal recording layer, and voltage applicationis stopped at the time when the calculated voltage has reached thethreshold value of the liquid crystal recording medium.
 6. A method forrecording information by using an information recording medium in whicha transparent electrode, a photoconductive layer and a liquid crystalrecording layer are formed on a transparent substrate with a transparentelectrode formed on said liquid crystal recording layer, or an integraltype of information recording medium in which a transparent electrode, aphotoconductive layer, a dielectric middle layer and a liquid crystalrecording layer are formed on a transparent substrate in this order andan additional transparent electrode is provided, and applying voltagebetween both electrodes of said information recording medium forexposure to image-carrying light, so that the liquid crystals areoriented to record the information, characterized in that the currentflowing through said liquid crystal recording layer is monitored tothereby calculate the voltage applied on said liquid crystal recordinglayer, and the duration of the applied voltage is controlled on thebasis of the calculated voltage.
 7. A method according to claim 6,characterized in that an electrode is formed on a part of saidphotoconductive layer, and an electric circuit element corresponding tosaid liquid crystal recording layer and dielectric middle layer isconnected to said electrode, so that when the voltage is applied, thecurrent flowing through said electric circuit element is monitored tocalculate the voltage applied on said liquid crystal recording layer,and the duration of the applied voltage is controlled on the basis ofthe calculated voltage.
 8. A method for recording information accordingto claim 7, wherein a pole is formed on a part of the photoconductivelayer, an electric circuit element corresponding to the liquid crystalrecording layer and to the dielectric intermediate layer is connected tosaid pole, the electric current flowing through the electric circuitelement when voltage is applied is monitored to calculate the voltageapplied on the liquid crystal recording layer, and voltage applicationis stopped at the time when the calculated voltage has reached thethreshold value of the liquid crystal recording layer.
 9. A methodaccording to claim 6, characterized in that an electrode is formed on apart of said dielectric middle layer, and an electric circuit elementcorresponding to said liquid crystal recording layer is connected tosaid electrode, so that when the voltage is applied, the current flowingthrough said electric circuit element is monitored to calculate thevoltage applied on said liquid crystal recording layer, and theapplication of the voltage is stopped when the calculated voltagereaches the threshold voltage.
 10. A method for recording informationaccording to claim 9, wherein a pole is formed on a part of a dielectricintermediate layer, an electric circuit element corresponding to theliquid crystal recording layer is connected to said pole, the electriccurrent flowing through the electric circuit element when voltage isapplied is monitored to calculate the voltage applied on the liquidcrystal recording layer, and voltage application is stopped at the timewhen the calculated voltage has reached the threshold value of theliquid crystal recording layer.
 11. A device for recording informationby oppositely facing across an air gap a photoelectric sensor having atransparent electrode and a photoconductive layer formed on atransparent substrate in this order to a liquid crystal recording mediumhaving a transparent electrode and a polymer dispersion type of liquidcrystal recording layer formed on a transparent substrate in this order,and applying voltage between both the electrodes for exposure toimage-carrying light, so that the liquid crystals are oriented to recordthe information, characterized by including a current-monitoringresistance connected between a power source for applying voltage betweenboth said electrodes and said information recording medium, a means formonitoring the current flowing through said liquid crystal recordinglayer from the voltage applied on said resistance and for calculatingthe voltage applied on the liquid crystal recording medium, and acontrol means for controlling the duration of the applied voltage on thebasis of the calculated voltage.
 12. A device according to claim 11,characterized in that an electric circuit element corresponding to saidliquid crystal recording layer is connected in series with a circuitcomprised of a photoelectric sensor and a current-monitoring resistance.13. A device according to claim 11, characterized in that acurrent-measuring circuit connected with a current-monitoring resistanceis connected with a power source for applying a bias voltagecorresponding to the discharge voltage of an air gap.
 14. A deviceaccording to claim 11, characterized in that an electrode is formed on apart of the surface of said liquid crystal recording layer, and voltageis applied on said liquid crystal recording layer to measure thecurrent, thereby measuring the electric properties of said liquidcrystal recording layer.
 15. A device according to claim 11,characterized in that a mask is formed on a part of the surface of saidphotoelectric sensor to be exposed to light, and the current value ofthe masked area is measured.
 16. A device according to claim 15,characterized in that the electrode of said photoelectric sensor or saidliquid crystal recording medium at the masked area is formed separatelyfrom the electrode of the portion exposed to light.
 17. A device forrecording information by using an information recording medium in whicha transparent electrode, a photoconductive layer, and a liquid crystalrecording layer are formed on a transparent substrate in this order, andan integral type of information recording medium in which a transparentelectrode, a photoconductive layer, a dielectric middle layer and aliquid crystal recording layer are formed on a transparent substrate inthis order and an additional transparent electrode is provided, andapplying voltage between both the electrodes for exposure toimage-carrying light, so that the liquid crystals are oriented to recordthe information, characterized by including a current-monitoringresistance connected between a power source for applying voltage betweenboth said electrodes and said information recording medium, means formonitoring the current flowing through said liquid crystal recordinglayer from the voltage applied on said resistance and for calculatingthe voltage applied on the liquid crystal recording medium, and acontrol means for controlling the duration of the applied voltage on thebasis of the calculated voltage.
 18. A device according to claim 17,characterized in that an electric circuit element corresponding to saidliquid crystal recording layer, or to said liquid crystal recordinglayer and said dielectric middle layer is connected in series with acircuit comprised of a photoelectric sensor and a current-monitoringresistance.
 19. A method for recording information by oppositely. facingacross an air gap a photoelectric sensor having a transparent electrodeand a photoconductive layer formed on a transparent substrate in thisorder to a liquid crystal recording medium having a transparentelectrode and a polymer dispersion type of liquid crystal recordinglayer formed on a transparent substrate in this order, and applyingvoltage between both the electrodes for exposure to image-carryinglight, so that the liquid crystals are oriented to record theinformation, characterized in that the current flowing through saidliquid crystal recording medium is measured to thereby monitor theorientation behavior of the liquid crystal of said liquid crystalrecording layer and so control the duration of the applied voltage onthe basis thereof.
 20. A method according to claim 19, characterized inthat the application of the voltage of the unexposed portion iscalculated to reach the threshold of said liquid crystal recordingmedium.
 21. A method according to claim 19, characterized in that thetime at which the contrast between the light and dark portions reaches amaximum is monitored from the change-with-time of the current of theexposed portion to stop the application of the voltage.
 22. A methodaccording to claim 19, characterized in that the time at which thecontrast between the light and dark portions reaches a maximum ismonitored from the change-with-time of the current difference betweenthe exposed and unexposed portions to stop the application of thevoltage.
 23. A method according to claim 19, characterized in that thearea to be measured in terms of current is irradiated with light of agiven intensity, and the application of the voltage is stopped when thedifference in the integral value of the current between the exposed andunexposed portions reaches the quantity corresponding to the capacitychange of said liquid crystal recording medium.
 24. A method accordingto claim 19, characterized in that the behavior of said liquid crystalrecording medium at the exposed portion is monitored by thedifferentiation of the current value of the unexposed portion to controlthe application of the voltage.
 25. A method according to claim 19,characterized in that an electrode is formed on the surface of theliquid crystal recording layer of said liquid crystal recording medium,and voltage is applied on said liquid crystal recording medium prior torecording the information, so that the resistance value of said liquidcrystal recording medium is measured from the current value.
 26. Amethod according to claim 19, characterized in that an electrode isformed on the surface of the photoconductive layer of said photoelectricsensor, and voltage is applied on said photoelectric sensor prior torecording the information, so that the current value is measured.
 27. Adevice for recording information by oppositely facing across an air gapa photoelectric sensor having a transparent electrode and aphotoconductive layer formed on a transparent substrate in this order toa liquid crystal recording medium having a transparent electrode and apolymer dispersion type of liquid crystal recording layer formed on atransparent substrate in this order, and applying voltage between boththe electrodes for exposure to image-carrying light, so that the liquidcrystals are oriented to record the information, characterized byincluding a current-monitoring resistance connected between a powersource and said photoelectric sensor, means for monitoring the value ofthe current flowing through a circuit from the voltage applied on saidresistance, a means for calculating the value of the current flowingthrough a circuit from the voltage applied on said resistance and formonitoring the orientation behavior of the liquid crystal of said liquidcrystal recording layer, and a control means for controlling theduration of the applied voltage on the basis of the monitored result.28. A device according to claim 27, characterized in that a spacer islocated between said photoelectric sensor and said liquid crystalrecording medium to keep the air gap constant, and a current-detectingelectrode is formed on a part of said photoelectric sensor or saidliquid crystal recording medium on which said spacer is not found.
 29. Adevice according to claim 27, characterized in that a mask is formed ona part of the surface of said photoelectric sensor to be exposed tolight, and the current value of the masked area is measured.
 30. Adevice according to claim 27, characterized in that the electrode ofsaid photoelectric sensor or said liquid crystal recording medium at themasked area is formed separately from the electrode of the portionexposed to light.
 31. A device according to claim 29 or 30,characterized in that a current-monitoring area is formed on the portionto be exposed to light, and current-monitoring resistances are connectedto the masked area and the portion exposed to light.
 32. A deviceaccording to claim 31, characterized in that an optical system isprovided for irradiating the current-monitoring area of saidphotoelectric sensor exposed to a given quantity of light.
 33. A deviceaccording to claim 31, characterized in that the values of thecurrent-monitoring resistances connected to the masked area and theportion exposed to light are preset at those for correcting thedifference in area between said electrodes.
 34. A device according toclaim 33, characterized in that an electrode to be formed on thephotoelectric sensor or said liquid crystal recording medium at thecurrent-monitoring area of said photoelectric sensor exposed to light isseparated from the portion exposed to light.
 35. A device according toclaim 33, characterized in that said optical system is comprised of ahalf mirror for separating a part of said image-carrying light and alens element for condensing the light from said half mirror.
 36. Adevice according to claim 33, characterized in that said optical systemis one separate from the exposure system for emitting lightcorresponding to the quantity of light of the subject.